Considerable attention has been focused on women's mental health in the perinatal period and the subsequent impacts on children. Comparatively, we know much less about maternal depression at later time points and the potential implications for child mental health. The objective of this paper was to explore the association between maternal depression and child emotional/behavioural difficulties at 4 years postpartum, taking into account earlier episodes of perinatal depression. The Maternal Health Study is a prospective cohort study of 1,507 nulliparous women. Maternal depressive symptoms were assessed using the Edinburgh Postnatal Depression Scale (EPDS) in early pregnancy and at 3, 6 and 12 months postpartum and again at 4 years postpartum. Maternal depressive symptoms at 4 years postpartum were associated with significantly increased odds of child emotional/behavioural difficulties (odds ratio (OR) = 3.46, 95 % confidence interval (CI) = 2.21-5.43). This remained significant after adjusting for earlier episodes of perinatal depression and socio-demographic characteristics (OR = 2.07, 95 % CI = 1.18-3.63). We also observed a robust association between child difficulties at age 4 and measures of socio-economic disadvantage. Our findings suggest a pressing need to rethink current paradigms of maternal health surveillance and extend mental health surveillance and support to at least 4 years postpartum.