A delayed onset of lactation (OL) is likely to have a negative influence on breast-feeding outcomes. Thus, it is important to identify risk factors for delayed OL. We examined prospectively the association between stress during labor and delivery and OL in a cohort of urban Guatemalan women. Healthy women (n = 136) were recruited during the first stage of labor in the labor unit of a social security hospital in Guatemala City. Women provided salivary samples for cortisol determinations 3.2 +/- 3.2 h before birth and 1.7 +/- 1.9 h and 17.1 +/- 4.7 h after delivery. At the same time points women were asked to respond to a 17-item psychosocial stress and anxiety questionnaire. Women were called daily until OL occurred. Primiparous women had higher antepartum and early postpartum cortisol levels that were twice as high as those among multiparous women (P < 0.05). The parity differential in salivary cortisol concentrations vanished with time. General linear model multivariate analysis indicate that multiparous women (n = 77), irrespective of mode of delivery, had an earlier OL (P < 0.05) than primiparae who underwent emergency cesarean section deliveries (n = 11) (adjusted mean +/- SEM, 2.5 +/- 0.1 vs. 3.4 +/- 0.3 d postpartum, respectively). OL took longer to occur among multiparous women with prenatal salivary cortisol levels above (vs. below) the 40th percentile [3 +/- 0.2 (n = 28) vs. 2.4 +/- 0.2 d postpartum (n = 38), respectively; P = 0.02]. Thus, stress during labor and/or delivery is likely to be a significant risk factor for delayed OL in urban Guatemala.