Seventy-five menstruating women seeking contraceptive advice were randomly allocated to treatment with combined oral contraceptives containing either ethinyl estradiol 30 micrograms + levonorgestrel 150 micrograms (30/150), ethinyl estradiol 50 micrograms + levonorgestrel 125 micrograms (50/125) or ethinyl estradiol 50 micrograms + levonorgestrel 250 micrograms (50/250). The concentrations of cholesterol, phospholipids, high density lipoprotein (HDL)-cholesterol, HDL-phospholipids and triglycerides were determined prior to treatment and after one, three and six months of medication. Triglycerides increased by 18--42 per cent after one to six months of treatment with 50/125. The HDL-cholesterol and HDL-phospholipids were reduced by 10 per cent during 50/250 medication. No other parameters showed any consistent alteration in any of the treatment groups. Raised triglyceride concentration and/or decreased HDL concentration may increase the risk for cardiovascular disease. It is therefore suggested that in order not to alter the HDL concentration a combined oral contraceptive agent should probably not contain more 19-nortestosterone type of progestogen 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.