An open multicenter trial was performed in six centers in Finland to study the efficacy, safety and acceptability of a new biphasic oral contraceptive pill containing natural estradiol and cyproterone acetate. The participants were 288 women with a mean age of 39.3 +/- 3.4 years (range 30-49) who were willing to use the new pill as their only contraceptive method. In total, 23% of the women were smokers. The cumulative experience was 2800 treatment cycles during the first year. The net 12-month continuation rate was 63%. One pregnancy occurred in a woman who lost 5 tablets in the second treatment cycle, which gives a 12-month cumulative pregnancy rate of 0.4%. Serum progesterone values, determined twice during the third treatment cycle, showed ovulation inhibition in 95% of women. There were no serious side effects. Intermenstrual bleeding was recorded by 35.5% and 24.5% of women at 3 and 12 months, respectively. The bleedings became scantier in most women and dysmenorrhoea disappeared. No changes were observed in total and high density lipoprotein cholesterol concentrations after 1 year. With the exception of intermenstrual spotting, the efficacy, safety and acceptability of the new pill was almost as good as that of the modern low dose oral contraceptives. This is the first pill containing natural estradiol that has gained clinical acceptance and which can also be prescribed for smokers over 35 years old until the climacteric.