The number of uninsured US residents has fallen by more than 11 million since President Barack Obama signed his health care overhaul five years ago.
The Centers for Disease Control and Prevention said on Monday the figure would still leave about 37 million people uninsured.
However, it is the lowest level measured in more than 15 years.
According to two reports issued by the centres, the most dramatic change in the numbers took place during the first nine months of 2014. As the law's major coverage expansion was taking effect, the number of uninsured fell by 7.6 million.
Monday was the law's fifth anniversary and supporters and detractors again clashed over its impact.
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Obama said the law in many ways was "working even better than anticipated", while House Speaker John Boehner said it amounted to a "legacy of broken promises".
House Republicans filed a lawsuit against the signature health care law and unilateral actions in November, citing a broad and systematic overreach of executive authority.
Top House Democrat Nancy Pelosi dismissed what she described as a "meritless" action, claiming it was the work of "impeachment-hungry extremists".
