Previous studies on effects of postpartum depression (PPD) on infant-mother attachment have been divergent. This may be due to not taking into account the effects of stable difficulties not specific for depression, such as maternal personality disorder (PD). Mothers (N=80) were recruited for a longitudinal study either during pregnancy (comparison group) or eight weeks postpartum (clinical group). Infants of mothers with depressive symptoms only or in combination with a PD diagnosis were compared with infants of mothers with no psychopathology. Depression and PD were assessed using self-report and clinical interviews. Infant-mother attachment was assessed when infants were 13 months using Strange Situation Procedure (SSP). Attachment (in)security was calculated as a continuous score based on the four interactive behavioral scales of the SSP, and the conventional scale for attachment disorganization was used. PPD was associated with attachment insecurity only if the mother also had a PD diagnosis. Infants of PPD mothers without co-morbid PD did not differ from infants of mothers with no psychopathology. These results suggest that co-existing PD may be crucial in understanding how PPD impacts on parenting and infant social-emotional development. Stable underlying factors may magnify or buffer effects of PPD on parenting and child outcomes.