The treatment of endometriosis with danazol or lynestrenol decreased cholesterol content of plasma high-density lipoproteins by 50% and 32%. There were simultaneous increases in low-density lipoprotein cholesterol content of 51% and 19%. We suggest that changes induced by lynestrenol are due to progestational activity, while in the case of danazol both antigonadotropic and androgenic activities are involved.