ACA Insurers Still Reject 1 in 5 Claims, Leaving Millions With Unpaid Bills

Affordable Care Act (ACA) insurers approved a higher share of claims in 2024 after years of stagnation, but the improvement remains modest. Nearly one in five in-network claims (19.1%) were still denied, leaving millions of Americans with unpaid medical bills despite having coverage.

MoneyGeek analyzed comprehensive claims data from all ACA Marketplace insurers reporting to CMS for plan year 2024. Denials vary widely, with rejection rates topping 25% in some markets while others approve more than 95% of claims.

Open enrollment for 2026 is underway. Understanding which insurers deny the most claims helps consumers choose better coverage.

From 2021 through 2023, the national denial rate held in a 20% to 23% band with no meaningful movement. The 3.4 percentage point drop in 2024 (a 15% relative decline) marks the first year the trend broke downward.

States with limited insurer competition continue to post the highest rejection rates, and the gap between best and worst remains wide. Alabama cut its denial rate by 15 points, the largest state improvement in 2024. Montana and North Dakota each dropped more than 10 points.

Five states went the other direction. Alaska's five-point climb put it at the second-highest denial rate in the country.

Major Insurers Show Mixed Performance

Blue Cross Blue Shield of Montana posted the largest improvement, dropping 27 percentage points from 39.4% to 12.5%. UnitedHealthcare's 15-point decline is more consequential given its volume of 6.4 million claims. Blue Cross Blue Shield of Alabama improved 16 points, falling from 34.8% to 19%.

Oscar Health and Molina Healthcare went the other direction. Oscar Health's seven-point increase brought it to 25.3%, the highest denial rate of any major national insurer. Molina Healthcare added three points to cross the 20% threshold. The 10 largest carriers as a group still improved, cutting their combined average from 23.2% to 19.4%.

At the state level, denial rates in 2024 spanned 5% to 27%, a fivefold gap between the best and worst performers. Hawaii and Alaska are at the top, and South Dakota and North Dakota are at the bottom. Insurer count doesn't determine outcomes: Florida and Texas both exceeded the national average with 11 and 14 competing carriers in their markets.

Among the 10 largest carriers, denial rates ranged from 18% to 25%, matching the variation seen at the state level. Oscar Health denied one in four claims at 25%. UnitedHealthcare had the highest claim volume at 6.4 million and denied 20%, a rate that would have been 34% a year earlier.

*Blue Cross and Blue Shield parent companies from different states operate independently and are separated in this analysis.

Administrative issues and coverage disputes made up nearly two-thirds of 2024 rejections. Medical necessity determinations were the basis for fewer than one in four. Put another way, 77% of denied claims were turned away not because a doctor said no, but because of paperwork or how the plan is designed.

Insurers say administrative denials catch duplicate billing and fraud. Consumer advocates say the system penalizes patients who filed correctly and got rejected anyway because of errors on the insurer's side.

Of 8.8 million denied claims in 2024, very few consumers challenged denials through formal appeals:

• 0.2% of denied claims were appealed internally (approximately 17 appeals per 10,000 denied claims).
• 56% of those internal appeals were upheld by insurers.
• Fewer than one per 10 million denied claims reached external appeal with an independent reviewer.

Only 40% of Marketplace enrollees know they have a right to an independent external review, according to KFF's 2023 survey. Enrollees with ACA plans (34%) were less likely to know about external appeal rights compared to those with Medicare (58%) and Medicaid (45%).

Some denied claims get paid after resubmission without formal appeals. Persistence pays off even without the full appeals process.

Denied claims can delay care or create surprise medical bills, even for in-network services that patients reasonably believed were covered. When insurers reject claims, patients may owe thousands in unexpected medical expenses.

Denial rates vary by 500% across states and 40% across insurers, making plan selection during open enrollment a critical financial decision. Understanding which insurers and states deny the most claims helps consumers choose plans that will actually pay when they need care.

Premiums rose 20% nationally for 2026, but rate increases vary widely by state and insurer. When evaluating plans, weigh both upfront costs and how often insurers pay claims. A low premium means little if your insurer denies one in four claims. Coverage access remains a challenge, with 27 million Americans still uninsured as enhanced ACA subsidies face expiration in 2026.

Federal transparency rules have improved accountability since 2015, yet 2024's improvement marks only a partial recovery from persistently high denial rates. Nearly a decade of transparency requirements hasn't driven meaningful change in insurer behavior.

Some denied claims get paid after resubmission without formal appeals. Persistence pays off even without the full appeals process.

Compare denial rates before choosing a Marketplace plan during open enrollment to avoid insurers with high rejection rates.

MoneyGeek analyzed the Transparency in Coverage (TiC) 2026 Public Use File from CMS, covering Plan Year 2024 claims reported in 2025. The dataset includes Qualified Health Plan (QHP) filings from individual ACA Marketplace insurers, excluding Small Business Health Options (SHOP) and Stand-Alone Dental Plans (SADPs). Metrics reflect in-network, post-service claims only.

Data Selection

Every insurer reporting at least 1,000 claims in plan year 2024 is included, whether or not that insurer continued offering plans in 2025. This preserves the full claims experience of patients covered during 2024, including those whose insurer later left the Marketplace.

How We Calculated Rates

Three metrics were calculated for each insurer and state:

• Weighted denial rates: claims denied ÷ claims received, weighted by claim volume
• Median issuer rate per state: the middle value when insurers are ordered by denial rate
• National insurer averages: combined rates for insurers reporting across multiple states

All values are rounded to whole percentages. Plans with suppressed or incomplete reporting are excluded. CMS publishes claim counts as raw numbers; MoneyGeek converts them to thousands in tables (5,200 in a table = 5.2 million actual claims). Totals cover in-network, post-service medical claims only. Pharmacy and stand-alone dental claims are excluded.

Limitations

The analysis is limited to HealthCare.gov states. The 13 states that run their own marketplaces (California, New York, Massachusetts, Washington, Colorado, Connecticut, Maryland, Minnesota, Nevada, New Jersey, Pennsylvania, Rhode Island and Vermont) aren't included.

Insurers participating in HealthCare.gov states in 2024 but not in 2026 didn't provide claims denial information. Data are self-reported and not independently audited.

Claims initially denied but later resubmitted and approved don't count as denied. Parent company names were obtained by merging the 2025 QHP landscape file with Medical Loss Ratio submission data using HIOS plan identification numbers.