From 1965 to 1972, 19 patients with the Budd-Chiari syndrome were investigated. An underlying diagnosis was made in 10 cases, polycythemia rubra vera being the commonest etiology. Percutaneous liver biopsy confirmed the diagnosis in 18 patients, and in 14 the site of hepatic vein obstruction was defined or its presence suggested by hepatic venography. Hepatic scintiscanning showed predominant central localization of radiocolloid in 7 patients. In another 8 patients this appearance was present in association with other less specific features. These findings were correlated with inferior vena cavography in 13 patients in whom a characteristic narrowing and distortion of the vein throughout its intrahepatic course was noted. In 3 other patients, the vein was found to be occluded. Autopsy evidence in 6 patients suggests that the central concentration of radiocolloid on scintiscanning and the narrowing and distortion of the inferior vena cava were due to disproportionate enlargement of the caudate lobe. Additional studies indicated that the separate venous drainage of the caudate lobe may be preserved when the main hepatic veins are occluded and that hypertrophy of the caudate lobe occurs because of its relatively more efficient perfusion. Demonstration of the enlarged caudate lobe by scintiscanning and inferior vena cavography provides valuable diagnostic support for the Budd-Chiari syndrome.