Seventy-five menstruating women seeking contraceptive advice were randomly allocated to treatment with combined oral contraceptives containing either ethinyl estradiol 50 micrograms + levonorgestrel 250 micrograms (50/250), ethinyl estradiol 30 micrograms + levonorgestrel 150 micrograms (30/150) or ethinyl estradiol 50 micrograms + levonorgestrel 125 micrograms (50/125). The concentrations of cholesterol, triglycerides, phospholipids, high density lipoprotein (HDL)-cholesterol and HDL-phospholipids were determined after one, three and six months and compared to the mean of two determinations of the same parameters before medication. Triglycerides increased by 18--42 per cent after 1--6 months of treatment with 50/125. The HDL-cholesterol and HDL-phospholipids were reduced by 10 per cent during 50/250 treatment. No other parameters showed any consistent alteration in any of the treatment groups. Raised triglyceride concentration and/or decreased HDL concentration increases the risk for cardiovascular disease. It is therefore suggested that in order not to alter the HDL concentration a combined oral contraceptive agent should not contain more gestagen-androgen than corresponding to 125--150 micrograms of levonorgestrel. To avoid a rise of the triglyceride level the weight relation between levonorgestrel and ethinyl estradiol should be about 5:1.