Disinhibited attachment behavior is related to early institutional rearing and to later social maladaptation. It is also seen among infants reared at home whose mothers have histories of child maltreatment or psychiatric hospitalization. However, little is known about the maternal psychiatric diagnoses that might be associated with disinhibited behavior or the mechanisms through which maternal diagnosis might influence infant behavior. In the current study (N = 59), 2 maternal diagnoses, borderline personality disorder (BPD; n = 13) and depression (n = 15), were compared with a no diagnosis group (n = 31) on extent of infant disinhibited behavior. Disinhibited infant behavior was assessed at infant age of 12-18 months using the validated Rating of Infant-Stranger Engagement. Mother-infant interaction was coded using the Atypical Maternal Behavior Instrument for Assessment and Classification. Results indicated that infants of mothers with BPD were significantly more likely to be rated as disinhibited in their behavior toward the stranger compared with infants of mothers with depression and with no diagnosis. Disinhibited behavior was further related to the quality of mother-infant interaction, and maternal frightened/disoriented interaction partially mediated the effect of maternal BPD on infant disinhibited behavior. Disinhibited behavior among previously institutionally reared infants is relatively resistant to intervention after toddlerhood and is associated with maladaptation into adolescence. Therefore, high priority should be placed on understanding the developmental trajectories of home-reared infants with disinhibited behavior and on providing early assessment and early parenting support to mothers with BPD. (PsycINFO Database Record (c) 2019 APA, all rights reserved).