A federal appeals court in the United States has overturned about 100 death sentences in the states of Arizona, Idaho and Montana.
The Supreme Court rewrote the rules last year
The court, in San Francisco, said it had made the decision because the death penalty was imposed by judges instead of juries.
In 2002, the US Supreme Court ruled that only juries, not judges, could impose death sentences.
The federal judges voted eight to three that anyone who was given a death sentence by a judge should have it changed to life imprisonment.
"By deciding that judges are not constitutionally permitted to decide whether defendants are eligible for the death penalty, the Supreme Court altered the fundamental bedrock principles applicable to capital murder trials," Judge Sidney Thomas said after the ruling.
Colorado and Nebraska, where judges also hand down death sentences, are overseen by different appeals courts which have not yet ruled on the issue.
A spokesman for the National Coalition to Abolish the Death Penalty welcomed the decision, saying it had reinforced the principle that juries should decide matters of life and death.
This sentiment was echoed by a federal defence lawyer in Phoenix, Arizona, who welcomed the decision.
"This is fundamental justice," said Ken Murray.
But the BBC's Martin Turner in Washington says the decision to overturn the sentences is itself likely to be the subject of a new appeal to the Supreme Court.
Other issues were also opened up by the Supreme Court's original ruling, including what happens in four other states where juries offer advisory verdicts but judges pass sentence.
Legal experts expect it to be months - or even years - before the full effect of the Supreme Court's decision becomes clear.
By then, our correspondent says, the make-up of the Supreme Court itself could have changed and with it the fates of the prisoners on Death Row.