Ninety four women and their first born children took part in a longitudinal study of maternal mental health during pregnancy and after delivery. The children's cognitive functioning was assessed at age 4 using the McCarthy scales, without knowledge of the mothers' psychiatric history or current health. As expected girls performed slightly better than boys and children from middle class and professional families did better than children from working class homes, as did children whose mothers had achieved at least one A level at school. Significant intellectual deficits were found in the children whose mothers had suffered with depression, but only when this depression occurred in the first year of the child's life. Marital conflict and a history of paternal psychiatric problems were independently linked with lower cognitive test scores; together with a working class home background these were the only factors that contributed to the deleterious effect of maternal postnatal depression.