This prospective, double-blind, randomized study was conducted to compare the contraceptive reliability, cycle control, and tolerability of a 23-day versus a 21-day oral contraceptive regimen containing 20 microg ethinyl estradiol and 75 microg gestodene. Participants took trial medication daily for 28 days, either 23 tablets with active substances plus 5 placebo tablets or 21 tablets with active substances plus 7 placebo tablets. Contraceptive efficacy, cycle control, and tolerability were evaluated over a period of seven cycles. Efficacy data gathered from 4,878 treatment cycles (23-day regimen: 2,362 cycles; 21-day regimen: 2,516 cycles) were obtained from 703 participants (23-day regimen, n = 342; 21-day regimen, n = 361). Both preparations proved to be effective contraceptives and provided good cycle control. One pregnancy because of method failure was recorded in each treatment group. This resulted in a study Pearl Index of 0.5 for each treatment. For the 23-day regimen, 36.0% of participants reported at least one intracyclic bleeding episode during Cycles 2-4 (primary target) compared to 37.1% in the 21-day regimen. In the 23-day regimen group, intracyclic bleeding episodes were reported by 42.4% of the participants in Cycle 1 but only in 14% in Cycle 7 and in the 21-day regimen group by 44.6% in Cycle 1 and only 17.3% in Cycle 7. Overall, intracyclic bleeding was reported in 21.9% of the 23-day regimen cycles and in 22.7% of the 21-day regimen cycles.A greater number of 23-day regimen participants had shorter withdrawal bleeding periods than with the 21-day regimen. In significantly (p <0.0001) more cycles in the 23-day regimen group, participants reported withdrawal bleeding periods that lasted only 1-4 days compared to the 21-day regimen group. For the majority of the treatment cycles, the median number of bleeding days in the 23-day regimen group was 4 days and in the 21-day regimen group 5 days. Both preparations were well tolerated and showed a similar adverse events pattern. The discontinuation rate because of adverse events was low (23-day regimen, 6%; 21-day regimen, 4%). No serious vascular adverse events were reported. More than 75% of the women in both groups either lost more than 2 kg of weight or did not gain weight during the study. The treatment effect on blood pressure was negligible. There were no appreciable changes in mean laboratory values over the course of the study.