Officials in the United States say as many as 7 million Americans will have signed up for President Barack Obama's healthcare programme by Monday's deadline.
The White House says that is a remarkable turnaround for the controversial programme, which launched in October 2013. Hundreds of thousands of Americans have now signed up to register for coverage and avoid a penalty next year.
Mr Obama's allies say the Affordable Care Act will bring healthcare to those who need it most, but Republicans believe it is a massive waste of money, the BBC reports.
The 2010 law, known as the Patient Protection and Affordable Care Act, is intended to extend health insurance to the roughly 48 million Americans who do not receive it through their employers, the government, or a privately purchased plan.
It also aims to slow the growth in the cost of healthcare through various measures and requires private plans to meet a certain level of coverage.
The initial launch of the system last year was dogged by website crashes and technical problems.
The law has proved deeply unpopular among many voters, and Republicans are expected to tap into that discontent in November's mid-term elections, that will determine the shape of Congress for Mr Obama's last two years in office.
The Republican Party and conservatives in the private sector see it as an inappropriate government intrusion into the massive healthcare industry and an affront to personal liberty.
Although 31 March was officially the last day people could sign up, millions could be eligible for extensions, including people who had begun enrolling but did not finish before the deadline as a result of errors or website glitches.
Extensions may also be available to those prevented from enrolling because of a natural disaster, for example, or because they were subjected to domestic abuse.