Josh Hartnett at New Armani store opening on 5th Avenue. New York City, USA - 17.02.09

The Black Hawk Down star checked into the hospital on Sunday (29Mar09) after a "flare-up" of an ongoing stomach problem that first affected him when he was performing on London's West End stage last year (08).

Hartnett's publicist reveals the actor has undergone tests all week - but doctors have yet to properly diagnose his condition.

Susan Patricola tells Usmagazine.com. "He has had a lot of testing. They released him, and they are trying to figure out what it might be. He is fine."

Source: WENN