The actor was taken to a hospital in the early hours yesterday (March 30) with extreme abdominal pains. The star had similar health issues last November when he was starring in Rain Man in London and was forced to take some time off from work to rest.
"He is under observation but is resting comfortably," said Hartnett’s rep.
The actor, who was staying at the Chateau Marmont in Los Angeles, was treated at Cedars-Sinai Hospital.