David Ranta's lawyer tells The New York Times 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 in the 1990 killing of a Brooklyn rabbi.
Brooklyn prosecutors had recently concluded Ranta's prosecution in the death of Rabbi Chaskel Werzberger was fatally flawed.
Werzberger was killed by a bandit fleeing a botched robbery. One witness said a police lineup that helped convict Ranta had been rigged.
Ranta is 58. He told reporters Thursday that his new freedom was emotionally overwhelming.