Behavioural, cognitive and emotional dysregulation related to parent borderline personality disorder (BPD) may have a major impact on parent-infant interactions. We investigated the ways in which infant social withdrawal and mother sensitivity may be associated to a BPD diagnosis in mothers. Two groups of clinic-referred mother-infant dyads were set up: one in which consulting mothers were diagnosed with BPD. Infant social withdrawal and maternal sensitivity were assessed during free-play interactions videotaped within a clinical setting. Mothers with BPD showed lower maternal sensitivity, while their infants displayed higher levels of social withdrawal behaviour when compared to dyads with no maternal psychiatric diagnosis. Infants of mothers with BPD appear at particular risk of adverse developmental outcomes from their very first months of life. Early interventions should concentrate on improving maternal interpretation and response to infant cues, such as identifying nonverbal behaviours and regulating the infant's emotional states, and on promoting mentalization.