The efficacy and safety of a low-dose 21-day combination oral contraceptive containing 100 microg levonorgestrel and 20 microg ethinyl estradiol were evaluated in an open-label, multicenter trial. A total of 1708 subjects with regular menstrual cycles (27,011 cycles) were evaluated. The oral contraceptive was administered once a day for 21 days, followed by 7 days of placebo for a complete cycle. During 26,554 cycles evaluated for efficacy, 18 pregnancies occurred (Pearl index of 0.88); 6 of these events were attributable to subject noncompliance. After 30 cycles of exposure the cumulative rate of withdrawal as a result of accidental pregnancy was 1.9%. Breakthrough bleeding (with or without spotting) occurred in 12.9% of the cycles and spotting alone occurred in 10.1% of the cycles. The 2 most common adverse events cited as reasons for discontinuation were headache (2% of subjects) and metrorrhagia (2%). One serious event led to withdrawal of a subject. Overall, the results of this study demonstrate that the monophasic regimen of 100 microg levonorgestrel and 20 microg ethinyl estradiol offers effective contraception, acceptable cycle control, and a good tolerability profile.