A patent umbilical vein is a sonographic sign of portal hypertension. The specificity of the sign is not known. In a review of 12 patients with sonographically demonstrated patency of the umbilical vein, we found two patients in whom no clinical evidence existed to suggest the presence of portal hypertension. In six of 10 healthy subjects, the lumen of the proximal umbilical vein measured up to 3 mm in diameter. A review of the literature indicates that in normal injection-corrosion studies, a tiny vessel can be seen in non-cirrhotic livers. A patent umbilical vein greater than 3 mm must be demonstrated, at least throughout the course of the intrahepatic portion of the ligamentum teres, to confirm portal hypertension.