Objective: To determine the risk factors, clinical features, and outcome of infants diagnosed with portal vein thrombosis (PVT). Study design: A retrospective chart review was conducted of all consecutive infants admitted to the Hospital for Sick Children, Toronto, between January 1999 and December 2003 diagnosed with PVT. Results: PVT was diagnosed in 133 infants, all but 5 of whom were neonates, with a median age at time of diagnosis of 7 days. An umbilical venous catheter (UVC) was inserted in 73% of the infants and was in an appropriate position in 46% of them. Poor outcome, defined as portal hypertension or lobar atrophy, was diagnosed in 27% of the infants and was significantly more common in those with an initial diagnosis of grade 3 PVT and in those with a low or intrahepatically placed UVC. Anticoagulation treatment did not appear to have a significant effect on outcome. Conclusions: PVT occurs early in life; major risk factors in addition to the neonatal period are placement of UVC and severe neonatal sickness. Poor outcome is associated with an improperly placed UVC and with grade 3 thrombus.