A 6-MONTH-OLD BOY was referred to our hospital for evaluation of a left upper quadrant abdominal mass. The infant was born of an uneventful pregnancy. Findings from several routine checkups after birth were normal. A few days before admission to the hospital, the mass, detected during a routine physical examination, was thought to be splenomegaly. There was no history of abdominal pain or fever. Physical examination disclosed an egg-sized, clearly definable, smooth, nontender mass in the left upper quadrant of the abdomen. The mass was unmovable.