To evaluate the contraceptive effectiveness of male latex condoms, we assembled a cohort of 300 women relying on male latex condoms for contraception and followed them prospectively for one menstrual cycle. None of the 234 women who completed the study and reported at least one act of intercourse conceived (one cycle pregnancy rate 0%; 95% CI 0%--2%). Given the pattern of intercourse during the cycle and 3 different sets of conception probabilities for different days of the cycle, we would have expected between 32 and 36 pregnancies if no condoms had been used. Thus, the prevented fraction due to condom use (effectiveness) is 100% and the lower bounds of the 95% CI are between 89% and 90%. For this cohort, male latex condoms were an effective method of contraception. This cost-efficient approach of evaluating contraceptive effectiveness shows promise for post-marketing studies.