The share of adults who skipped medical care because of costs dropped by nearly one-fifth between 2013 and 2015, according to a report from the Commonwealth Fund.
The gains were even more dramatic in the states that have most expanded coverage through the federal healthcare law, often called Obamacare.
"The Affordable Care Act's health insurance coverage expansions are working to get people covered and help them afford healthcare," said Sara Collins, vice president of the fund, a New York-based foundation that studies healthcare systems around the world.
"We know that health insurance is essential for people to get the care they need," Collins said. "In this time of uncertainty about the ACA, it's important to keep in mind the financial and health protection health insurance provides for families."
Healthcare costs have risen sharply in recent years for some Americans, particularly those making too much money to qualify for government assistance.
Last year, 13% of adults reported that they went without care because of the cost.
That was down from 16% in 2013, a year before the debut of the healthcare law's system of guaranteeing coverage to Americans, even if they are sick.
It remains unclear how Republicans plan to replace Obamacare. GOP leaders have not developed legislation that would protect the gains in coverage and access.
Since 2014, Obamacare has made hundreds of billions of dollars available to states to extend
Medicaid eligibility historically was limited to certain vulnerable populations, including low-income children, pregnant women, people with disabilities and the elderly.
Obamacare also has offered subsidies to low- and moderate-income Americans to help them buy private health coverage on new insurance marketplaces such as HealthCare.gov.
Nationwide, this system has brought health coverage to more than 20 million previously uninsured Americans and driven the nation's uninsured rate to historic lows.
At the same time, residents of 38 states and the District of Columbia were less likely in 2015 than in 2013 to say costs prevented them from seeing a doctor when they were sick, the new report found.
The biggest declines were in states that expanded Medicaid through the law, an option that has been taken by 31 states.
Kentucky, which saw the largest decline in uninsured, also saw the most dramatic gains in access. The percentage of adults in the state who reported skipping care because of costs plunged from 19% in 2013 to 12% in 2015.
Also recording big gains were Arkansas and Oregon, which each saw a 5-percentage-point drop in adults who skipped care.
In California, the share of adults with cost-related access problems dropped by a quarter, from 16% to 12%, according to the report.
Researchers found that patients had the most trouble affording care in states that have resisted Obamacare the most, primarily in the South.
In Mississippi, 19% of adults reported skipping care in 2015 because of cost. In Texas, it was 18%. And in Alabama and Florida, the rate was 17%.
Elected leaders in all four states have rejected the Medicaid expansion and have been among the most vocal supporters of calls to repeal Obamacare.
By contrast, fewer than one in 10 adults reported cost-related access problems in states with the most affordable medical care, led by Iowa.