OBJECTIVE To study use of oral contraceptives among Finnish teenagers during 1981-91 and how abortions, childbirths, sexually transmitted diseases, and cardiovascular diseases changed during this period. METHODS Biannual cross sectional surveys with mailed questionnaires from 1981 onwards and analysis of national statistics. METHODS Finland. METHODS A nationwide sample of 14, 16, and 18 year olds. Sample size varied from 1249 to 3887 and response rate from 85% to 94%. METHODS Proportion taking oral contraceptive, fertility and abortion rates, hospital discharge rates, rates of sexually transmitted diseases. RESULTS The proportion of teenagers taking oral contraceptives increased steadily. In 1991 the percentages among 14, 16, and 18 year olds were 2%, 18%, and 41% compared with 0.2%, 7% and 22% in 1981. Most users had a steady partner (80% of 16 year olds and 85% of 18 year olds). By 1989 rates of abortion had fallen from 12/1000 to 9.3/1000 in 16 year olds and from 25/1000 to 19.2/1000 in 18 year olds; fertility rates had fallen from 4.5/1000 to 2.3/1000 and from 23.5/1000 to 15.3/1000 respectively. Rates in 14 year olds fell only slightly. Gonorrhoea infection fell and HIV infection remained rare. Rates of hospital discharge after thromboembolic venous disease rose slightly. CONCLUSIONS The increased use of oral contraceptives is the most likely explanation for decreasing abortion and fertility rates among teenagers. Increased reliance on the condom because of the threat of AIDS may increase unwanted pregnancies.