As part of a longitudinal study of the influence of postnatal depression on child development, the cognitive functioning of index and control children was assessed at age 5 years. There was no evidence of an adverse effect of postnatal depression, even amongst sub-groups of children suggested to be vulnerable (boys and children from low SES families). However, early experience of insensitive maternal interactions predicted the persistence of poorer cognitive functioning. A number of factors in the child's current environment, including stimulation at home, social class and, for boys, the experience of schooling, contributed to cognitive performance. The findings are considered in relation to the ongoing debate on sensitive periods.