Cortisol levels dramatically increase during pregnancy, peak at birth, and subsequently decline. However, all previous studies examined women during pregnancy and early postpartum. None examined the long-term association of parity and lactation with cortisol levels. We examined the relation of reproductive history to cortisol levels in postmenopausal women. Subjects were 749 women, aged 50-89, who were not using estrogen in 1984-1987 when morning cortisol was measured. Parity was not significantly associated with cortisol. However, women who breast-fed for >12 months had significantly higher cortisol levels than women who breast-fed for shorter durations or not at all (p = 0.003). This association was stronger among women with three or more births. Duration of breast-feeding is a determinant of cortisol levels in postmenopausal women. Because both increased cortisol and increased duration of breast-feeding may play protective roles in certain autoimmune diseases, such as rheumatoid arthritis, we suggest that the beneficial effect of lactation on the course of these diseases may be mediated by cortisol.