EY-Parthenon Emerges as Largest Strategy Practice in Big Four

Inside Ernst & Young

LONDON — Ernst & Young reported $53.2 billion in global revenue for the fiscal year ending June 2025, a 4.0 percent increase in local currency. EY is the third-largest of the Big Four professional services firms, behind Deloitte and PwC. The firm employs more than 406,000 people across 150 countries and territories.

EY’s primary service lines are Assurance, Consulting, Strategy and Transactions, and Tax. Strategy and Transactions services, now operating under the EY-Parthenon brand following a 2025 reorganization, is the firm’s deal advisory platform. The brand is one of EY’s four global service lines and the only EY sub-brand to retain its acquired name after a decade of integration.

EY was formed in 1989 through the merger of Ernst & Whinney and Arthur Young & Co. The firm rebranded from Ernst & Young to EY in 2013. In FY25, AI-related revenue grew 30 percent year over year, with more than 15,000 EY professionals working on AI-led client projects across the consulting and EY-Parthenon service lines. EY appeared on Fortune magazine’s 100 Best Companies to Work For list in 2026 for the 28th consecutive year, the longest active streak among major U.S. accounting firms.

The 2025 EY-Parthenon Reorganization

EY-Parthenon began as The Parthenon Group, an independent strategy consulting firm founded in 1991 by former Bain & Company directors Bill Achtmeyer and John Rutherford. EY acquired the Boston-based firm in 2014, adding roughly 350 strategy consultants to its then-Transaction Advisory Services practice. The deal was widely read as EY’s answer to PwC’s 2013 acquisition of Booz & Company and Deloitte’s 2013 purchase of Monitor Group.

In 2025, EY moved its entire Strategy and Transactions service line under the EY-Parthenon brand. The combined practice now numbers roughly 25,000 professionals across 150 countries, making it the largest strategy consulting arm of the Big Four. By comparison, Monitor Deloitte has about 6,000 staff, PwC’s Strategy& about 4,500, and KPMG’s Global Strategy Group about 4,000.

PE sponsors engaging EY-Parthenon for ABA platform diligence now access commercial due diligence specialists, financial due diligence analysts, operational assessment teams, and corporate strategy consultants within a single organizational unit. The firm’s Strategy and Transactions service line generated $6.2 billion in revenue in FY25, flat in local currency from FY24. EY-Parthenon was the only one of EY’s four service lines to record no revenue growth in the fiscal year.

For ABA buyers whose investment thesis depends on de novo growth and payer expansion, an integrated commercial-and-financial diligence model carries different cost and coordination implications than a fragmented multi-firm approach.

Due Diligence Service Lines

EY-Parthenon provides financial, tax, commercial, operational, human resources, AI, IT, product and technology, sustainability, cyber, and transaction tax due diligence. The firm’s diligence approach is designed to identify transaction value drivers, improve deal structures, and challenge assumptions about future performance. Buy-side and sell-side teams use the same analytical infrastructure.

The firm’s Advanced Analytics team supports diligence work through data quality assessments, customer segmentation analysis, pricing optimization, and network performance evaluation. For ABA transactions, that capability can address patient census trends, payer mix stability, geographic concentration risk, BCBA retention and supervision ratios, and clinical outcome metrics. Quantitative diligence is the firm’s competitive differentiator against advisors that rely primarily on traditional financial statement analysis.

On the sell side, EY-Parthenon provides vendor due diligence, separation advisory, and carve-out support. The firm was ranked Mergermarket’s #1 global M&A accountant advisor in 2025 by both deal value and deal volume, including in Australia, France, Germany, Japan, Singapore, and the United States. The ranking reflects accountant-advisor work specifically; investment-bank league tables track different transactions.

Healthcare and Behavioral Health Practice

EY’s healthcare advisory practice serves providers, payers, pharmaceutical companies, and health technology companies. EY-Parthenon advises PE sponsors on healthcare provider acquisitions, including commercial diligence on market trends, competitor positioning, and operational efficiency.

EY does not maintain a dedicated behavioral health practice in the way some competitors do. Its healthcare transaction advisory team engages with behavioral health platforms through PE advisory work, but the firm’s organizational structure is functional rather than industry-specific. Healthcare expertise is distributed across practices rather than concentrated in a single branded unit. PE sponsors should request healthcare-experienced team members when engaging EY-Parthenon for ABA transactions.

EY-Parthenon’s HR due diligence team assesses organizational structure, talent management, compensation benchmarking, and workforce risks. In ABA, where clinical workforce shortages constrain capacity, those factors drive revenue sustainability and growth: clinician retention, BCBA supervision ratios, RBT (registered behavior technician) turnover, and credentialing compliance. Workforce dynamics are an operational dimension that often separates a buyable platform from a fragile one.

The firm’s IT and cyber due diligence practice evaluates electronic health record systems, practice management platforms, billing and revenue cycle technology, and data security posture. For ABA platforms where EHR fragmentation across acquired practices often creates operational drag, this work can identify integration costs that affect post-acquisition economics. Tech diligence has grown more central as ABA operators adopt telehealth supervision and AI-assisted clinical documentation tools.

Where EY-Parthenon Fits in ABA Deals

EY-Parthenon’s strongest fit in ABA M&A is for PE sponsors who want financial and commercial diligence handled by a single team. The combined model suits platform acquisitions where the buyer’s thesis depends on growth assumptions, like de novo clinic openings, geographic penetration, and payer diversification, that need commercial validation alongside financial verification.

On purely financial diligence questions, where the issue is whether historical EBITDA (earnings before interest, taxes, depreciation, and amortization) is sustainable and whether identified adjustments are reasonable, EY-Parthenon competes with Deloitte, PwC, and KPMG at the large-cap end and with RSM, BDO, and Grant Thornton in the middle market. Its fee structure is consistent with Big Four pricing. Mid-market ABA transactions typically engage EY-Parthenon for commercial or strategic diligence layered on top of financial QofE work performed by a mid-market specialist.

The firm’s AI investment is a factor in its diligence approach. EY invests more than $1 billion annually in AI platforms and products, including 1,000 AI agents and more than 100 internal applications. For ABA transactions involving large datasets, like multi-state Medicaid claims, billing records, and clinical documentation, AI tools can accelerate pattern recognition and anomaly detection in ways that traditional review cannot match.

“The winning CEOs are no longer waiting for global stability. They are moving with confidence to acquire capabilities, specifically AI and next-generation technology, that are rewiring their businesses for resilience and driving portfolio transformation.” — Mitch Berlin, EY Americas Vice Chair, EY-Parthenon (October 2025)

What ABA Operators and Sponsors Should Know

For ABA practice owners, EY-Parthenon enters the picture when a large PE sponsor or strategic acquirer engages the firm for diligence. Owners should expect commercial diligence to evaluate not just historical financial performance but the defensibility and scalability of the clinical model, market positioning, and growth trajectory. That level of scrutiny rewards documentation of clinical outcomes, market share data, competitive dynamics, and strategic growth plans.

For PE sponsors, EY-Parthenon’s integration of strategy and transactions reduces coordination costs, compresses timelines, and produces unified diligence findings. The trade-off is fee structure and the absence of a branded ABA practice, which means sponsors must vet team-specific behavioral health experience before signing an engagement letter.

EY-Parthenon’s October 2025 Deal Barometer projects U.S. PE deal volume will rise 5 percent in 2026, following an estimated 8 percent increase in 2025. Corporate M&A volume is forecast to grow 3 percent in 2026. Healthcare services is among the sectors EY tracks for elevated separation activity. Whether the firm’s expanded model translates into deeper share of ABA-specific mandates is the open question. The next Deal Barometer update is due in mid-2026.

AT A GLANCE

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