OBJECTIVE To determine the relation between infant feeding practices (and other factors) and the duration of postpartum amenorrhea, and to establish whether there are real differences in the duration of postpartum amenorrhea for similar breast-feeding practices in different populations. DESIGN Prospective, nonexperimental, longitudinal follow-up study. SETTING Five developing and two developed countries. PATIENT(S) Four thousand one hundred eighteen breast-feeding mothers and their infants. INTERVENTION(S) Breast-feeding women collected ongoing information about infant feeding and family planning practices, plus the return of menses. Fortnightly follow-up occurred in the women's homes. MAIN OUTCOME MEASURE(S) A multivariate analysis explored the association between the risk of menses return and 16 infant feeding variables and 11 other characteristics. RESULT(S) Ten factors (in addition to center effects) were significantly related to the duration of amenorrhea. Seven of these were infant feeding characteristics and the remaining three were high parity, low body mass index, and a higher frequency of infant illness. CONCLUSION(S) The breast-feeding stimulus is strongly linked to the duration of postpartum amenorrhea. Cross-cultural effects also are extremely important and may have caused the variations in feeding, the variation in amenorrhea, or both.