Mothers with borderline personality disorder (BPD) have been theorized to have decreased mentalization ability, which is the capacity to perceive and interpret mental states. This could increase the risk for troubled relationships with their infants and therefore have adverse consequences for child social and emotional development. Mind-mindedness (MM), which codes the mother's references to her infant's mental states during an interaction, is one method of indexing mothers' mentalizing ability. However, research has yet to examine MM in mothers with BPD. Our objective was to assess the MM ability of 38 mothers during interactions with their 12-month-old infants, including 10 mothers with BPD and 28 mothers without a psychiatric diagnosis. Trained observers assessed maternal MM from 2 min of videotaped mother-infant free play. BPD was assessed with the Structured Clinical Interview for DSM-III-R-Personality Disorders (SCID-II). Mothers with and without BPD did not differ in the proportion of total comments referring to infant mental states. However, mothers in the BPD group proportionately made 3.6 times more misattuned mind-related comments than control mothers. Thus, mothers with and without BPD appear equally likely to envision mental states in their infants. However, mothers with BPD also appear more likely to misread their infants' mental states.