Twelve patients with proved Budd-Chiari syndrome (eight acute and four chronic cases) were examined, using real-time ultrasonography (US). In all acute cases, US study showed at least one hepatic vein with findings suggestive of the syndrome, such as stenosis, dilatation, thick wall echoes, thrombosis, abnormal course, or extrahepatic anastomosis. In chronic cases, hepatic veins were usually not visible. Modifications of liver morphology were present in all patients except those with recent onset of the disease. Caudate lobe hypertrophy was present in only six cases. US study is therefore the procedure of choice for initial diagnosis of acute Budd-Chiari syndrome. Pitfalls were the failure to detect two caval thromboses and one hepatic vein web. Cavography should still be performed systematically, but hepatic phlebography is useful in selected cases only.