The accuracy of ultrasound assessment of portal vein patency has been defined by comparing it with the results of arterial portography in 115 cases. The accuracy of arterial portography was confirmed in 21 cases where orthotopic liver transplantation was performed and used as a 'bench-mark' against which to assess the ultrasound findings. Ultrasound correctly assessed portal vein patency in 87.5% of patients. It was more accurate in assessing patency (90%) than occlusion (68%). Ultrasound correctly assessed portal vein patency in 90% of cases of cirrhosis and hepatic malignancy. Difficulties occurred in children with biliary atresia particularly following the Kasai operation (37.5% accuracy). In the absence of previous surgery to the portal vein or biliary system, ultrasound is comparable to arterial portography and can be used as the sole means of assessment.