Previous data of lipid and lipoprotein level changes with the application of danazol, an isoxazol derivative of 17-alpha-ethinyl testosterone, have been conflicting. We measured cholesterol, triglycerides, low density lipoproteins and high density lipoproteins, estradiol and progesterone in 62 patients treated for endometriosis with danazol 600 mg daily. The fasting blood samples were taken before, every month during danazol medication, and 4, 12 and 24 weeks after cessation of therapy. Inconsistent changes in total cholesterol and triglyceride levels were observed. The data showed a constant, though not significant increase of the mean LDL levels. Plasma HDL decreased approximately 45% during the first two months of danazol influence and remained constantly low for the rest of the treatment. The atherogenic ratio (LDL:HDL) was doubled by-danazol. Five patients developed a reversible type IIa hyperlipoproteinemia. The plasma levels of estradiol decreased, but showed normal midfollicular values during the treatment period. In contrast, the plasma levels of progesterone fell significantly and were sometimes undetectable. These findings demonstrate an atherogenic potential of danazol, especially when long term treatment is taken into consideration.