To determine the effects of oral contraceptives on lipids and Iipoproteins over a six-month period, we randomized 266 women into four oral contraceptive groups: ethinyl estradiol 35 µ/g plus ethynodiol diacetate 1 mg, ethinyl estradiol 30 µg plus levonorgestrel 0.15 mg, ethinyl estradiol 35 µg plus norethindrone 1 mg, and ethinyl estradiol 35 fig plus norethindrone 0.5 and 1 mg (biphasic). For all groups, total cholesterol increased 5.9-9.1% from baseline values over the six months. Triglycerides increased with all preparations, with the ethynodiol diacetate group (37.6%) and the biphasic norethindrone group (45.3%) showing the greatest increase. Low-density lipoprotein cholesterol increased 10-15.6% among the groups; low-density lipoproteinapolipoprotein B changed proportional to the low-density lipoprotein cholesterol increases. All groups except the ethynodiol diacetate group showed a decrease of highdensity lipoprotein cholesterol, with the levonorgestrel group (8.7%) and biphasic norethindrone group (4.5%) showing the largest declines. ApolipoproteinA-l increased in all groups, with the ethynodiol diacetate preparation (19.3%) showing the greatest increase and the levonorgestrel preparation (3.2%) showing the smallest increase from baseline values. The changes in apolipoproteinA-l were out of proportion to the changes in high-density lipoprotein cholesterol, suggesting that the high-density lipoprotein particle may be undergoing some type of metabolic alteration.