A follow-up study was carried out to investigate the children of 204 mothers who had previously participated in a study of their mental health during pregnancy and the first postnatal year. One hundred and seventy two children, 170 mothers, and 99 fathers were assessed when the children were 3 years 10 months. Boys of mothers depressed in the first year postpartum scored approximately 1 standard deviation lower on standardised tests of intellectual attainment than boys whose mothers were well that year. The difference was reliable even when behaviour during the test was controlled for, and general behavioural problems, birth weight, parental IQ measures of the family climate and home environment, mother-child interaction, and breast-feeding during infancy were taken into account.