John Ritter, best known for his role as Jack on the TV series Three's Company, died after collapsing on the set of his current ABC series, Eight Simple Rules.
His publicist says no one knew he had a heart defect that led to what's called an "aortic dissection." That's a rare medical condition which is often fatal and sudden, when the body's main artery enlarges, then tears.
The American Heart Association says the problem is sneaky because the only real clue is high blood pressure. For some reason, the rupture occurs most often between the ages of 50 and 55. John Ritter was 54.