Thirty-one patients with malignancy, anticoagulated for the treatment of venous thromboembolism (VTE) are reported. Complications of treatment included major bleeding (35%), redevelopment of VTE whilst on therapeutic levels of anticoagulants (13%) and recurrent VTE following cessation of anticoagulant (42%). Forty-two per cent of patients died within three months of initiating anticoagulant therapy. The cause of death was progressive malignancy, except in one patient who died of anticoagulant-related bleeding. The high incidence of bleeding is frequently associated with over-anticoagulation and indicates the need for improved monitoring. The frequent recurrence of VTE is associated with chronic disseminated intravascular coagulation, for which warfarin is ineffective and heparin is indicated.