Objective: To determine the etiologic factors and outcome of acute pancreatitis in children under age 3 years. Study design: This was a retrospective study of children under age 3 years with acute pancreatitis between January 1995 and December 2004. Stringent diagnostic criteria were used. Demographic and clinical data were collected, and etiology and outcome were recorded. The study was approved by the University of Pittsburgh's Institutional Review Board. Results: Of 109 cases, 87 met the diagnostic criteria. Median age was 20 months (range, 1 week to 35 months). AP was associated with multisystem disease in 29 cases (34%), with hemolytic uremic syndrome (HUS) being common. Pancreatitis was associated with systemic infections in 16 cases (18%) and was idiopathic in 15 cases (17%). Biliary disease played an important etiologic role (9%), as did trauma (8%). Pancreatitis was mild in 76 cases (87.3%) and severe in 3 cases (3.4%). Conclusions: AP is commonly associated with multisystem disease, particularly with HUS. Idiopathic pancreatitis and pancreatitis associated with biliary disease are seen in children under age 3 years. Trauma is a less frequent cause of pancreatitis, and severe pancreatitis is rare in this age group.