The Budd-Chiari syndrome caused by occlusion of the major hepatic veins, often of unknown etiology, is typically characterized by massive ascites, hepatomegaly and abdominal pain due to intense congestion of the liver. The outcome has almost always been fatal. This report describes an evaluation of side-to-side portacaval shunt in dogs with experimental Budd-Chiari syndrome and in six patients with hepatic vein thrombosis. In the animal studies, side-to-side portacaval shunt was very effective in relieving massive ascites, hepatomegaly, hepatic congestion and portal hypertension produced by ligation of the hepatic veins. Only one of 24 dogs with side-to-side anastomosis reformed ascites, 67% of the animals survived until the study was concluded after one year, and liver biopsies showed reversal of the severe pathologic abnormalities. In contrast, all 20 control dogs subjected to a sham laparotomy, and all 20 dogs that underwent end-to-side portacaval shunt reformed massive ascites and died within six months with continued hepatic congestion and necrosis. All six patients with the Budd-Chiari syndrome due to hepatic vein occlusion had massive ascites (4.4-15.9 l), hepatomegaly, abdominal pain and disturbed liver function. In all six, angiography demonstrated occlusion of the hepatic veins with a patent inferior vena cava (IVC) and a normal IVC pressure, and liver biopsy showed intense centrilobular congestion and necrosis. The most valuable diagnostic study was angiography of the IVC and hepatic veins with pressure measurements. Side-to-side portacaval shunt was performed from four to 14 weeks after the onset of symptoms, and produced dramatic and sustained relief of ascites in five of the six patients during follow-up periods of from eight months to seven years. Liver function returned to normal, hepatosplenomegaly disappeared, none of the survivors developed portal-systemic encephalopathy, and follow-up liver biopsies showed disappearance of congestion and necrosis, but mild to moderate fibrosis. One patient died following an emergency IVC thrombectomy and portacaval shunt, which was undertaken when, during the course of his workup, his condition deteriorated suddenly because the thrombotic process extended from the hepatic veins into the IVC. The everpresent risk of this complication, and the dangers associated with delaying operation were emphasized by this case. It is concluded that side-to-side portacaval shunt, which decompresses the liver by converting the portal vein into an outflow tract, provides effective treatment of the Budd-Chiari syndrome when the occlusive process is confined to the hepatic veins.