Plasma levels of betathromboglobulin (BTG), fibrinopeptide A (FPA) and B beta 15-42 fragment, indices of platelet release, thrombin generation and plasmin activity respectively, were measured in 32 high risk patients during a double blind study of a single dose of the anabolic steroid stanozolol (50 mg IM) in the prevention of DVT after major gastro-intestinal surgery. The prevalence of malignancy and the incidence of DVT (125I fibrinogen scan) were similar in the two treatment groups. On the first postoperative day, BTG, FPA and B beta 15-42 levels were increased in most patients. Plasma BTG levels were significantly increased on the first post-operative day in patients who developed a DVT (n = 14) compared to those patients who did not (n = 18). A significant increase in FPA levels was found in the DVT group, 7 days after surgery. On the morning before surgery, plasma B beta 15-42 levels were significantly increased in patients who developed a DVT. In patients undergoing surgery for early malignancy (n = 17), we observed a pre-operative increase in FPA levels when compared to patients without malignancy. At post-operative day 7, B beta 15-42 levels were significantly increased in patients who received stanozolol (n = 15), when compared to the placebo group, suggesting that intramuscular stanozolol increases fibrinolysis in vivo.