The synthetic steroid, danazol [17 alpha-pregna-2,4-dien-20-yno-(2,3,d)isoxazol-17-ol], is used widely for the treatment of endometriosis. In nine subjects studied over a 6-month course of treatment and for 5 months subsequently, plasma high density lipoprotein cholesterol (HDL-C) levels fell during treatment (P less than 0.001) and were restored to pretreatment levels within 3-5 months after treatment ceased. In these patients, total cholesterol did not increase significantly, yet the depressed levels of HDL-C apparently masked an increase in the total cholesterol in the remaining lipoprotein classes (P less than 0.01). Triglyceride levels were not affected significantly over the course of the treatment. Lipoprotein electrophoresis indicated an elevated beta-lipoprotein band in the subjects using danazol. All effects of danazol on plasma levels of cholesterol and HDL-C were reversed within 3-5 months after the cessation of treatment.