This study tests the hypothesis that maternal depression during pregnancy predicts temperament in offspring aged 6 m to 5 y. Previous studies have shown that maternal depression is related to negative affect and that certain temperament factors, such as negative affect and behavioral inhibition, in children predict affective disorders. Here, maternal depression is divided into depression during pregnancy vs. depression postpartum. Maternal depression was determined by the Beck Depression Inventory (BDI) throughout pregnancy and postpartum (prospectively) and by a diagnostic interview (SCID) at 6 months postpartum. The data show that maternal depression during pregnancy, but not postpartum, predicted the ratings of negative affect in the offspring. Importantly, symptoms of depression in the mother (BDI) were used as a control variable in the analyses in order to control for potential bias related to the mother's mood. In addition, cortisol levels in response to a mild stressor at 6 months of age predicted negative affect in infants and toddlers. We conclude that the effects of maternal depression on behavioral problems and vulnerability to mental illness may be mediated by altered temperament and enhanced stress responsiveness.