In twenty-seven patients with venography-documented Budd-Chiari syndrome (BCS), underlying diseases included polycythemia vera (nine patients), membranous occlusion of the inferior vena cava (three), and cirrhosis (two). In only nine patients did the syndrome occur in the absence of any predisposing factor. All patients with a membranous web were relatively young (mean age 29 years) and were born in Morocco. With regard to prognosis, three groups could be delineated. Eight patients formed a rapidly progressive group, with mean survival from first symptom of only 4.3 weeks, and were characterized mainly by their relatively advanced age and minimal enlargement of the spleen.