The aim of the study was to examine long-term sequelae in the children of mothers who were depressed at 3 months postpartum. In a community sample from two general practices in South London, 149 women were given psychiatric interviews at 3 months postpartum and 132 of their children (89%) were tested at 11 years of age. The children of women who were depressed at 3 months postpartum had significantly lower IQ scores. They also had attentional problems and difficulties in mathematical reasoning, and were more likely than other children to have special educational needs. Boys were more severely affected than girls, with the sex difference most pronounced on Performance IQ. The links between postnatal depression and the children's intellectual problems were not mediated by parental IQ and were not accounted for by measures of social disadvantage nor by the mother's later mental health problems. Breastfeeding did not remove the effect of the mother's illness on Full Scale IQ, but exerted its own influence on Verbal IQ and appeared to mediate the link with mathematical ability. The findings show that adverse experiences in infancy predict cognitive ability and academic performance a decade later.