David Ranta's lawyer tells The New York Times that the former inmate had a serious heart attack Friday night and is being treated at a New York hospital.
Ranta walked out of jail Thursday after a judge threw out his conviction for murdering a Brooklyn rabbi in 1990.
Brooklyn prosecutors had recently concluded that Ranta was innocent in the death of Rabbi Chaskel Werzberger, who was killed by a bandit fleeing a botched robbery.
The trial had featured falsified witness testimony and a rigged lineup.
Ranta is 58. He told reporters Thursday that his new freedom was emotionally overwhelming.