The purpose of this study was to establish the safety and efficacy of sodium warfarin in the secondary prophylaxis of venous thrombosis in patients with cancer. This was an inception cohort study of patients enrolled in an anticoagulation clinic between July 1991 and October 1996. The rates of bleeding and recurrent thrombosis were evaluated in all the patients, and the results in patients with cancer (n = 104) were compared with those without cancer (n = 208). The rate of major hemorrhage was 0.4% and 0.3% per treatment month in the patients with cancer and those without cancer, respectively. The rates of recurrent thrombosis were 1.2% and 0.2% per treatment month in the patients with cancer compared with those without cancer, respectively. We conclude that warfarin is safe when used for the secondary prophylaxis of patients with cancer who have had a venous or arterial thrombosis, and the risk of major hemorrhage is not significantly different when compared with the risk in patients without cancer. The rate of recurrent thrombosis is approximately sixfold higher in patients with cancer being treated with warfarin for secondary prophylaxis of thrombosis compared with patients without cancer. Nonetheless, the rate of recurrent thrombosis is not overly excessive, and warfarin can be viewed as a relatively effective form of therapy for these patients.