Two monophasic oral contraceptives containing gestodene (GTD, 75 micrograms) and ethinylestradiol (EE, 30 micrograms) or norgestimate (NGS, 250 micrograms) and EE (35 micrograms) were compared during the first six cycles of use. The subjects were randomly assigned to receive either type: 97 received GTD/EE and 92 NGS/EE. Six women in the GTD/EE group and nine in the NGS/EE group withdrew from the study; three (3%) and two (2%), respectively, withdrew because of adverse reactions. A total of 562 cycles for GTD/EE and 523 for NGS/EE were available. No woman became pregnant during the study. Overall, 94.4% of cycles in the GTD/EE group and 92.8% in the NGS/EE group were normal. A similar incidence of breakthrough bleeding (0.2% of cycles for GTD and 1.6% for NGS) and spotting (5.4% vs. 5.6%) was observed. Amenorrhea was never reported. Duration of withdrawal bleeding tended to be slightly longer in the NGS/EE group, significantly so for cycles 2 (0.5 days, p = 0.016), 4 (0.5 days, p = 0.031) and 5 (0.4 days, p = 0.045). Cycle 2 was significantly longer in the GTD/EE group (0.3 days, p = 0.027). Side-effects were reported by 12 (12%) women in the GTD/EE group and 13 (14%) in the NGS/EE group. The most common side-effects were headache (five cases (5%) in the GTD/EE group and two (2%) in the NGS/EE group) and breast pain (three (3%) and eight (9%) cases respectively). There were no statistically significant differences between the two groups with respect to change in body weight or changes in blood pressure and in laboratory data.(ABSTRACT TRUNCATED AT 250 WORDS)