Over a six-month period, 62 endometriosis patients were given 600 mg per day of the ethisterone derivative danazol. Blood count and coagulation status were checked before and during treatment. There was a significant increase (p less than 0.05) in hemoglobin and hematocrit. There were no changes in RBC, leukocyte count, or thrombocyte count; the mean cellular erythrocyte volume, thrombin time, thromboplastin time, and partial thromboplastin time also remained unchanged, as did factors VII, VIII, X, XII, and alpha-1-antitrypsin. Antithrombin III levels increased, while alpha-2-macroglobulin values decreased. Only the drop in fibrinogen, to pathologic values, and the increase in plasminogen reached significant levels (p less than 0.01). These in part contradictory changes suggest that hypocoagulability occurs under danazol medication; however, its clinical relevance is unclear.