Development and behaviour at five years were assessed in children from a national cohort; comparisons were drawn between those living in one-parent families, in step-parent families and with both natural parents. On average, children from one-parent families scored worst and children from two-parent families best on tests of behaviour, vocabulary and visuomotor co-ordination. The differences remained significant after allowing for associated influences. The behaviour of children from one-parent families and step-parent families was perceived on average as more 'anti-social' than that of children from two-parent families. Children in one-parent families were seen on average as marginally more 'neurotic' than children in two-parent families.