BACKGROUND The antiphospholipid-antibody syndrome is a thrombophilic disorder in which venous or arterial thrombosis, or both, may occur in patients with antiphospholipid antibodies. The optimal treatment of these patients is unclear. We assessed the efficacy of warfarin, low-dose aspirin, or both in the secondary prevention of thrombosis in patients with the syndrome. METHODS One hundred forty-seven patients (124 [84 percent] of whom were female) with the antiphospholipid-antibody syndrome and a history of thrombosis were studied retrospectively. The syndrome was primary in 62 patients and was associated with systemic lupus erythematosus in 66 patients and lupus-like disease in 19. Each patient's history was reviewed. RESULTS One hundred one patients (69 percent) had a total of 186 recurrences of thrombosis. The median time between the initial thrombosis and the first recurrence was 12 months (range, 0.5 to 144 months). Treatment with high-intensity warfarin (producing an international normalized ratio of > or = 3) with or without low-dose aspirin (75 mg per day) was significantly more effective (P < 0.001 by the log-rank test) than treatment with low-intensity warfarin (producing an international normalized ratio of < 3) with or without low-dose aspirin or treatment with aspirin alone in preventing further thrombotic events (recurrence rates per patient-year, 0.013, 0.23, and 0.18, respectively). The rate of recurrence of thrombosis was highest (1.30 per patient-year) during the first six months after the cessation of warfarin therapy. Complications involving bleeding occurred in 29 patients during warfarin therapy and were severe in 7 (0.071 and 0.017 occurrence per patient-year, respectively). CONCLUSIONS The risk of recurrent thrombosis in patients with the antiphospholipid-antibody syndrome is high. Long-term anticoagulation therapy in which the international normalized ratio is maintained at or above 3 is advisable in these patients.