The Obama administration has said barely 27,000 Americans enrolled for health insurance through its troubled federal website in the first month.
About 106,000 people were insured in total, most of them through the state-run websites.
The administration originally estimated nearly half a million people would sign up in the first month.
Democrats reportedly expressed frustration about the botched rollout in a White House meeting on Wednesday.
The federal website, used in 36 US states, has been bedeviled by glitches since its 1 October launch.
The administration has pledged that the portal will be "running smoothly" for a "vast majority" of users by the end of November.
Wednesday's figures from the US health department also showed nearly 400,000 Americans had qualified for Medicaid, a government medical programme for the poor that was expanded by the healthcare law.
About 40% of people in this category were said to have come through the federal website.
Nearly one million people, meanwhile, had managed to check via the website if they were eligible for government subsidies towards the insurance, but had not selected a plan, according to the administration.
The White House's chief technology officer, Todd Park, told a congressional oversight hearing on Wednesday that the system's response times have improved.
But there is a long way to go - health insurance enrolment thus far is a tiny fraction of the seven million people the Obama administration has projected will sign up by the end of March.
The difficult launch of the website has provided Republicans with plenty of ammunition against the law, which they tried to delay or defund through a partial government shutdown last month.
The White House is also facing harsh criticism over insurance companies' mass cancellation of policies that do not meet the law's strict requirements, even though Democratic President Barack Obama had pledged otherwise.
Adding to his political headache, six Senate Democrats are sponsoring a bill that would allow Americans to hold on to their existing coverage. The proposal is entitled Keeping the Affordable Care Act Promise.
It is along the lines of a House Republican-sponsored bill, to be voted on on Friday, which Democratic leaders have rejected as just another conservative attempt to repeal the law.
Democrats in the House of Representatives met White House officials on Wednesday, reportedly to express their concern that the issue could lead to a backlash in next year's midterm elections.
An unnamed senior House Democratic aide told US media that legislators had pressed Mr Obama to announce a fix for the cancelled policies.
The White House is expected to host a visit from Democratic senators on Thursday.
Former President Bill Clinton, a fellow Democrat, on Tuesday urged Mr Obama to find a way to let Americans keep their coverage under the law. Republicans seized on his comments.
Last week, Mr Obama apologised to those whose policies had been cancelled, saying "we didn't do a good enough job in terms of how we crafted the law".
A recent US poll suggested that for the first time more than half of Americans do not trust the president.