A 5-MONTH-OLD male infant was admitted to the hospital with a 3-day history of fever associated with anorexia, fussiness, and decreased sleep. Information from a review of systems and history of present illness revealed a several-week history of constipation and gastroesophageal reflux. On physical examination the infant appeared healthy and had a nontender, firm, abdominal mass palpated in the right upper quadrant. The mass extended 9 cm below the costal margin. By auscultation, bowel sounds were present, and a diffuse low-pitched bruit was heard over the mass. Findings from the remainder of the physical examination were unremarkable.