The value of ultrasonography was evaluated in 85 dogs and 17 cats presented with a clinically suspected portosystemic shunt (PSS). A PSS was confirmed in 50 dogs and nine cats (single congenital extrahepatic in 42, single congenital intrahepatic in 11, and multiple acquired in six). Six dogs and one cat had hepatic microvascular dysplasia, and 29 dogs and seven cats had a normal portal system. Ultrasonography was 92% sensitive, 98% specific, and had positive and negative predictive values of 98% and 89%, respectively, in identifying PSS, with an overall accuracy of 95%. When a PSS was identified with ultrasonography, extrahepatic, intrahepatic, and multiple acquired PSS could be correctly differentiated in 53/54 patients (98%). The combination of a small liver, large kidneys, and uroliths had positive and negative predictive values of 100% and 51% for the presence of a congenital PSS in dogs. The portal vein/aorta (PV/Ao) and portal vein/caudal vena cava (PV/ CVC) ratios were smaller in animals with extrahepatic PSSs compared with animals with microvascular dysplasia, intrahepatic PSSs and those without portal venous anomalies (P<0.001). All dogs and cats with a PV/Ao ratio of < or = 0.65 had an extrahepatic PSS or idiopathic noncirrhotic portal hypertension. Dogs and cats with PV/Ao and PV/CVC ratios of > or = 0.8 and > or = 0.75, respectively, did not have an extrahepatic PSS. Reduced or reversed portal flow was seen in four of four patients with multiple acquired PSSs secondary to portal hypertension. The presence of turbulence in the caudal vena cava of dogs had positive and negative predictive values of 91% and 84%, respectively, for the presence of any PSS terminating into that vein.