Danazol is known to cause marked suppression of sex hormone binding globulin (SHBG) levels in plasma and to increase the proportion of plasma testosterone unbound to protein but the effect on the concentration of total and free testosterone is unclear. Twenty-five patients with endometriosis were treated daily for 6 months with doses of danazol ranging from 50 to 600 mg. The fall in SHBG and rise in percent free testosterone was dose-related during the early part of treatment. Suppression of total testosterone and 5 alpha-dihydrotestosterone levels occurred and was probably due to increases in metabolic clearance rates. The observed fall in androstenedione levels was related to the incidence of menstrual abnormality, suggesting that this might be due to reduced ovarian activity. The concentration of free testosterone increased by a factor of two in the first week but subsequently returned to levels of between 25 and 50% above pretreatment levels. This pattern of changes may be due to the rise in metabolic clearance rates being dependent on induction of enzymes of androgen metabolism.