OBJECTIVE The aims of this study were to present the prevalence, imaging and clinical findings, and possible causes of portal venous system aneurysms. MATERIALS AND METHODS From 1998 to 2006, a total of 38 portal venous system aneurysms identified in 25 patients were retrospectively reviewed. The data of seven patients diagnosed using color Doppler sonography or CT before March 2004 were not consecutive, but the data recorded thereafter comprised the analysis of 4,186 consecutive patients who underwent routine abdominal MDCT. The patients were 14 men and 11 women (mean age -/+ SD, 53 -/+ 17 years). RESULTS The prevalence of portal venous system aneurysm among 4,186 consecutive patients was 0.43%. There were no differences with respect to patient age, patient sex, and intrahepatic or extrahepatic location of aneurysm between those with and those without portal venous system aneurysm. Seven of the 25 patients with portal venous system aneurysm were symptomatic because of portal vein thrombosis, and six of them had recurrence. These patients had significantly larger aneurysms than those without symptoms. Four of those seven symptomatic patients evaluated for thrombophilia had an underlying defect. Other associated findings were splenomegaly (n = 16), portal hypertension (n = 8), cirrhosis (n = 3), psoriasis (n = 2), portal vein variation (n = 1), chronic pancreatitis (n = 1), and cutis laxa (n =1). CONCLUSION Portal venous system aneurysms were rare in our study group but occurred more frequently than previously thought. All thrombosed aneurysms, most with a recurrence, were symptomatic and larger in patients with symptoms of portal venous system aneurysm than in those without symptoms. There were no differences among patients with portal venous system aneurysm and those without portal venous system aneurysm with respect to patient age and patient sex. Among those with aneurysms, there was no difference between subjects with intrahepatic versus extrahepatic aneurysms. A thrombophilic defect probably played a role in development of thrombosis in the portal venous system aneurysm.