A 67-year-old man with adenocarcinoma of the colon had recurrent pulmonary emboli. The patient's level of serum antithrombin III was found to be below normal. Heparin therapy led to further reduction, resulting in failure of adequate anticoagulation. The patient subsequently died of massive pulmonary emboli despite heparin therapy. We believe the antithrombin III level should be routinely determined for patients receiving heparin therapy.