Demographic studies generally use menses to indicate the return of fertility during breastfeeding. In a prospective study of 40 breastfeeding women in Manila, the resumption of ovarian activity was monitored by urinary hormone assays, and menstrual status was determined by weekly interview. For the women who menstruated before six months postpartum, first menses was not a good indicator of ovulation because there was a high proportion of anovular menses (67 percent), and the lag between anovular first menses and subsequent ovulation was 15.7 (+/- 4.4) weeks. After six months postpartum the proportion of anovular first menses declined to 22 percent, and the lag between anovular first menses and ovulation was 7.3 (+/- 4.6) weeks. If all anovular and ovulatory menstrual episodes are considered, the mean interval between first observed menses and first ovulation was 8.4 weeks during the first six months postpartum and only 0.1 week after six months. Thus, in breastfeeding women, menses is an inaccurate proxy measure for the timing of fertility return before six months postpartum, but a good indicator of the resumption of ovulation after six months.