Local broadcaster KABC said the singer was taken by ambulance to the UCLA Medical Center.
The Los Angeles Times says he was not breathing when fire department paramedics arrived at his home.
The paramedics performed CPR and took him to the medical centre, says the newspaper. Officials did not confirm if the CPR was successful.
TMZ is now reporting that Michael has died, although this has yet to be confirmed by other sources.
The singer is due to begin a series of comeback concerts at London's O2 arena on 13 July.