The course of long-term oral anticoagulation in 25 patients (186 patient-months) with cancer managed in an outpatient anticoagulation clinic was analyzed to determine the frequency of complications. Major and minor hemorrhagic complications occurred in 10.7% and 32% of treatment courses, respectively, for an incidence of 1.6% and 4.8%, respectively, per patient-month. Recurrent thromboembolism occurred in 14% of treatment courses. These results are better than previous reports in the literature, but worse than our overall anticoagulation clinic experience. Patients with cancer requiring anticoagulation are at a higher risk of hemorrhagic and thrombotic complications. Accordingly, we recommend that such patients be monitored closely, preferably by physicians experienced with such therapy, as in an anticoagulation clinic, to avoid an excessive degree of morbidity.