A federal judge in the US state of Texas has ruled that a key part of the Affordable Care Act (ACA), better known as Obamacare, is unconstitutional.
Twenty states argued the whole law was invalidated by a change in tax rules last year which eliminated a penalty for not having health insurance.
President Donald Trump said the ruling was great news for America.
The law's provisions will, however, remain in place until an appeal is heard at the US Supreme Court.
President Trump promised to dismantle Barack Obama's landmark 2010 healthcare law, which was designed to make medical cover affordable for the many Americans who had been priced out of the market.
But despite his Republican Party having majorities in both the House of Representatives and the Senate, the ACA is still operating.
However, in 2017 Congress did repeal the requirement - the so-called individual mandate - that people buy health insurance or pay a tax penalty.
Mr Trump took to Twitter following the judge's ruling in Texas.