Some 22 million Americans could lose their health insurance over the next decade under a Senate bill to replace Obamacare, a congressional report says.
However, the bill would reduce the budget deficit, the non-partisan Congressional Budgetary Office said.
Similar legislation passed by the House was also said to leave millions uninsured. Some Republicans have voiced reservations about the plan.
But the White House disputed the CBO's figures.
Responding to Monday's report, it said: "The CBO has consistently proven it cannot accurately predict how healthcare legislation will impact insurance coverage".
The report is a review of draft legislation unveiled by the Republican party last week.
It is unlikely to be approved by Democrats, who see the proposals as cruel and unfair.
The CBO said that 15 million more people would be uninsured by 2018 under the proposed legislation than under current law, largely because the penalty for not having insurance would be eliminated.
President Donald Trump's Republican party is struggling to secure the 50 votes it needs to get its bill through the Senate when it comes to the floor.
On Thursday, Republicans Ted Cruz, Ron Johnson, Mike Lee and Rand Paul said they were "not ready to vote for this bill", but were "open to negotiation".
Their concerns about the measure include its rollback of the expansion of Medicaid, the public health program for the poor and disabled.
Mr Johnson said on Sunday there was "no way" there should be a vote on the legislation this week.
The Senate bill would slash taxes for the wealthy offering less help for working families to buy medical insurance.
Top Democrat Nancy Pelosi earlier warned that "hundreds of thousands" of Americans would die if congressional Republicans pass their healthcare legislation.
Republicans have rejected the notion that anyone will die as a result of their healthcare plan.