A longitudinal study was conducted to evaluate the ability of a low-dose triphasic oral contraceptive to suppress ovulation as documented by frequent ultrasonographic scanning and progesterone determinations, even in the event of a missed pill. The extent of follicular growth and maturation, the incidence of escape ovulation, and the effect of correct and incorrect pill intake were assessed in 30 evaluable women during two consecutive spontaneous menstrual cycles. After the first cycle, 11 of 30 women (36.6%) had follicle-like structures of at least 10 mm in diameter. Ten of 11 structures gradually disappeared during the second cycle, with one persistent structure remaining through the second cycle. Seven of 30 women (23%) developed follicle-like structures during the second cycle. Of these, one woman had a probable ovulation, and another had an elevated progesterone level without follicle rupture, suggesting the luteinized unruptured follicle syndrome. Both of these women missed a pill on day 1 of the second cycle. In all cases cervical scores indicating hostility were noted. Thus, although suppression of ovarian activity may have been incomplete when oral contraceptives were incorrectly taken, secondary mechanisms of contraception remained operant. When they were correctly taken, low-dose triphasic oral contraception consistently prevented ovulation.