Prolonged on-demand breast feeding is known to delay the resumption of postpartum ovarian cyclicity. At present, however, little is known about the factors that influence the effectiveness of breast feeding as a natural contraceptive. Here, I examine the effects of maternal nutritional status on the duration of postpartum amenorrhea in two socioeconomic groups of Au forager-horticulturalists of lowland Papua New Guinea. Although women in both groups continue to breast-feed their offspring for approximately three and one-half years, well-nourished wage-earning Au women experience their first postpartum menses just over one year earlier (median = 12.5 months) than their more poorly nourished traditional counterparts (median = 26.6 months). Probit analyses are used to demonstrate that, even after controlling for time since delivery, maternal age, parity, and supplementation of infants' diets, the duration of postpartum amenorrhea is significantly (p