A comparative clinical trial of two combined oral contraceptives differing only in estrogen type and dosage was conducted at the Centro de Investigaciones Hideyo Noguchi in Merida, Yucatan, Mexico. The trial was designed to determine the differences between Norinyl 1 + 50 (Syntex) and Norinyl 1 + 35 (Syntex) in rates of discontinuation and frequency of selected side-effects which might contribute to method discontinuation. Three hundred women were randomly assigned to either the Norinyl 1 + 35 group or to the Norinyl 1 + 50 group and follow-up visits were scheduled at 1, 4, 8 and 12 months after admission. In the Norinyl 1 + 35 group, more women experienced an increase in intermenstrual bleeding (primarily staining and spotting) (p less than 0.05), breast discomfort (p less than 0.05) and nausea than in the Norinyl 1 + 50 group. There was a significantly higher discontinuation rate for personal reasons, such as desired change of method and method not needed, among the women taking Norinyl 1 + 35 (p less than 0.05). The largest number of discontinuations comprised women discontinuing for menstrual problems in both groups. The life-table total discontinuation rate at 12 months was 52.0 for the Norinyl 1 + 35 group and 50.7 for the Norinyl 1 + 50 group. The lost-to-follow-up rates at 12 months were 17.8 for the Norinyl 1 + 35 group and 22.8 for the Norinyl 1 + 50 group.