To assess retrospectively three antithrombotic treatments in the secondary prevention of thrombosis in the antiphospholipid syndrome (APS), 23 patients (six systemic lupus erythematosus, seven lupus-like disease and 10 primary antiphospholipid syndrome) were included in this study. Treatments assessed were: (1) aspirin 75 mg daily, (2) warfarin (international normalised ratios (INRs) 2.0-2.9) +/- aspirin 75 mg daily, and (3) warfarin (INRs > 2.9) +/- aspirin 75 mg daily. Where patients had received two or three of these treatments successively, the periods of time on each treatment were added and the number of patients with recurrence(s) on each treatment were compared by Fisher's exact probability test. 'High' anticoagulation (INRs > 2.9) +/- aspirin 75 mg daily was more effective than aspirin 75 mg daily, there was a trend in favour of 'high' anticoagulation (P = 0.066). No statistically significant difference could be demonstrated when comparing 'low' anticoagulation +/- aspirin 75 mg daily with aspirin 75 mg daily (P = 0.092). These results suggest that aggressive anticoagulation with or without low-dose aspirin is effective in preventing further thromboembolic events in APS.