A San Francisco appeal court has overturned an estimated 100 death sentences because the inmates were sent to death row by judges instead of juries.
The case stems from a 2002 decision by the US Supreme Court, that juries, not judges, must pass death sentences. But the Supreme Court left unclear whether the new rules should apply retroactively to inmates awaiting execution.
By an 8-3 vote, the San Francisco court said all condemned inmates sentenced by a judge should have them commuted to life terms.
The ruling applies only to Arizona, Idaho and Montana.
Two other states, Nebraska and Colorado, have also allowed judges to sentence inmates to death, but the federal appeals courts that oversee them has yet to rule on the issue.
Defence lawyers welcomed the verdict.
"This is fundamental justice," said Mr Ken Murray, a public defender in Phoenix.
Mr Murray estimated the decision affected at least 100 inmates on Arizona's death row alone.
The case the court used to decide the issue concerned Arizona inmate Warren Summerlin. The body of Brenna Bailey (36), a finance company administrator, was found in the boot of her car a day after she visited Summerlin to check on money he owed. Summerlin was convicted in 1982 and a judge sentenced him to death.
AP