One hundred fifty-six cases of deep venous thrombosis were reviewed. A previous episode of thrombosis, seen in 47 percent of the patients, was the most common predisposing factor. Medical illness, operation and local trauma were other important predisposing factors. Treatment consisted of continuous intravenous heparin therapy monitored by the activated coagulation time of whole blood (ACT). Thirty-three patients with an average ACT greater than 180 seconds had a more rapid recovery and had no recurrent deep venous thrombosis, embolism or failure to improve. Recurrent thrombosis, embolism or failure to improve was seen in 7 of 65 patients with an average ACT below 180 seconds. Major bleeding, defined as life-threatening bleeding or bleeding requiring transfusion, was not increased when the ACT was over 180 seconds, although minor bleeding was increased. Bleeding was also increased in women, elderly and postoperative patients. We advocate the use of higher doses of heparin to treat most patients with deep venous thrombosis because of the more rapid and complete recovery.