Danazol was given to 16 patients with endometriosis in daily doses of 200 to 400mg for 4 to 6 months, and the change in serum lipids and lipoproteins was analysed. Danazol decreased total cholesterol by an average of 22%, triglycerides by 56%, phospholipids by 29%, free fatty acids by 17% and lipid peroxides by 42%, among which reductions in total cholesterol (p less than 0.05) and lipid peroxides (p less than 0.01) were statistically significant when compared with the pretreatment levels. Levels of lipids in HDL and VLDL also fell: HDL-cholesterol by 43%, HDL-triglycerides by 68%, HDL-phospholipids by 40%, VLDL-cholesterol by 73%, VLDL-triglycerides by 71% and VLDL-phospholipids by 48%. The decrease in HDL-cholesterol (p less than 0.05), HDL-triglycerides (p less than 0.05) and HDL-phospholipids (p less than 0.001) was significant. Concurrently apolipoproteins and LCAT activity decreased: A-I by 28% (p less than 0.001), A-II by 7% (p less than 0.05) and LCAT by 35%. In contrast, LDL-cholesterol increased, though slightly. Therefore, the atherogenic index defined as the ratio of LDL-c/HDL-c rose as much as 98% of the initial value. These changes seem to have arisen from impairment in the VLDL-HDL cycle. The decrease in VLDL and increase in LDL indicated that VLDL was catabolised and converted to LDL at a normal rate while the formation of HDL was impaired probably due to the low activity of LCAT and low level of apo A-I and A-II.(ABSTRACT TRUNCATED AT 250 WORDS)