Depression is common among women with young children, and is strongly associated with financial adversity. Debt is a common feature of such adversity, yet its relationship with depression has not been examined before. We have used longitudinal data, collected over six months, on 271 families with young children, to examine this relationship. Multiple regression was used to identify independent predictors of the total Edinburgh Post-natal Depression Scale score from a range of socioeconomic, demographic, social support and child health related variables. Worry about debt was the strongest independent socioeconomic predictor of the depression score at both initial and follow-up occasions. To account for the possibility of reverse causation, i.e. depression causing worry about debt, alternative regression models are reported which show that owing money by itself predicts depression and earlier debt worries predicts depression six months later. We were unable to show that earlier debt worries independently predicted subsequent depression scores after the initial depression score had been taken into account in the analysis. Although debt has not been shown to be an independent prospective predictor of depression, our results suggest it has a central place in the association between socioeconomic hardship and maternal depression. Evidence from qualitative studies on poverty and from studies on the causes of depression support this hypothesis. The implications for policy are that strategies to enable families to control debt should be an explicit part of wider antipoverty measures which are designed to reduce depression and psychological distress among mothers of young children.