Findings on effects of Postpartum depression (PPD) on infant-mother attachment have been contradictory. This may be due to not considering maternal interpersonal difficulties, for example co-morbid personality disorder (PD). We examined the role of PD in the association between postpartum depression and infant-mother attachment. Mothers were recruited either during pregnancy (non-clinical group, n=56) or eight weeks postpartum (PPD-group, n=29). Infants of mothers with PPD only or in combination with PD were compared with infants of mothers with no psychopathology. Depression and PD were assessed with questionnaires and clinical interviews. Infant-mother attachment was assessed when infants were 13 months using Strange Situation Procedure. Mothers with PPD were more likely to have co-morbid PD compared with non-clinical mothers. PPD was associated with attachment insecurity only if the mother also had co-morbid PD. Infants of mothers with PPD only did not differ from infants of non-clinical mothers. Our results suggest that PPD mothers, like any other clinical population, is a heterogeneous population and that co-morbid PD, may be crucial in understanding how PPD impacts on infant-mother attachment. Future research should focus on potential moderators of risk, as well as potential protecting factors when investigating effects of PPD on infant outcomes.