The influence of maternal stress during pregnancy on the postpartum iron status and immune maturation of infants was investigated in a nonhuman primate model. Forty infant rhesus monkeys were generated from two types of disturbed pregnancies, early or late gestation stress, and compared with 24 undisturbed controls. Prenatal stress increased the prevalence and magnitude of iron deficiency (ID) as the infants' growth-related demands for iron exceeded dietary intake from breast milk. At 4-6 mo of age, the emergence of ID significantly accentuated an effect of prenatal stress on natural killer cell activity, an important component of innate immunity. These findings indicate that maternal stress, especially early in pregnancy, should be added to the list of risk factors that warrant closer scrutiny of hematological profiles in fast-growing babies.