A large sample of primiparous women was screened for depression after childbirth. Those identified as depressed, women with a previous history of depression and a control group were followed up to 18 months, when their infants were assessed on measures of cognitive, social and behavioral development. Infants of postnatally depressed mothers performed worse on object concept tasks, were more insecurely attached to their mothers and showed more mild behavioural difficulties. Postnatal depression had no effect on general cognitive and language development, but appeared to make infants more vulnerable to adverse effects of lower social class and male gender.