In a prospective study of 22 patients with non-tumorous Budd-Chiari syndrome, four were found to have the antiphospholipid syndrome with no other cause of hepatic vein thrombosis. All four patients were young women. The antiphospholipid syndrome was secondary to systemic lupus in one case, to a "lupus-like disease" in another, and apparently primary in the remaining two cases. Two patients died. The other two are in good health on chronic oral anticoagulation. In our experience, the antiphospholipid syndrome is a frequent cause, after myeloproliferative disorders, of non-tumorous Budd-Chiari syndrome. In such patients, long-term anticoagulation may prevent recurrence or extension of thrombosis.