The estimated time of ovulation (ETO) was correlated with the day of defined postovulatory infertility in 66 hormonally normal menstrual cycles from 24 subjects for each of 15 different natural family planning methodologies. Inherent weaknesses were identified in methods based upon calendar calculations or basal body temperature only. These weaknesses could be removed for the basal body temperature- only methods if symptoms, especially the peak mucus symptom, were added to the temperature records. However, the peak mucus symptom alone had the greatest precision of all methods studied. No advantage could be identified in combining the basal body temperature with the peak symptom.