Objective: No previous longitudinal study has examined the impact of comorbid maternal personality disorder (PD) and depression on child development. We set out to examine whether maternal PD and depression assessed at 2 months post partum would be independently associated with adverse developmental outcomes at 18 months of age. Method: Women were recruited into the study shortly after delivery and screened for depression and PD. Those meeting criteria for depression, PD, or both conditions, were selected for assessment at 2 months post partum, together with a comparison group with neither condition (total sample, N = 200). Assessments of cognitive, social and emotional development were conducted with their children at 18 months of age. Results: Maternal postpartum depression and PD were both associated with higher levels of dysregulated infant behavior. There was a significant interaction between depression and PD in the model of dysregulated behavior and the detrimental effects of maternal depression and PD were evident only among mothers with both conditions. Maternal depression was independently associated with impaired infant cognitive scores and higher levels of internalizing behavior. Conclusions: Future studies of the effects of maternal depression should also take into account the effects of comorbid maternal PD. Health professionals need to be aware of the possible co-occurrence of PD among mothers presenting with postnatal depression and that mothers with these co-occurring disorders are likely to require greater support. (Contains 1 figure and 5 tables.)