Research has examined temperament in individuals with borderline personality disorder (BPD) but not in their offspring, despite offspring's risk of developing BPD and the importance of temperament in the etiology of BPD. We recruited a low-socioeconomic sample of 36 mothers with BPD and their children ages 4 through 7, and 34 normative comparisons. Replicating prior studies, mothers with BPD reported themselves as having more negative affectivity (frustration, fear) and less effortful control (inhibitory control, attentional control, activation control) than did comparisons. Mothers with BPD also reported that their offspring had more negative affectivity (anger/frustration, fear) and less effortful control (inhibitory control, attentional focusing) than did comparisons. We were concerned about potential bias and shared method variance. We therefore provided validity support for mothers' ratings of their children with teacher ratings of child behavior and child self-report via their story-stem completion narratives. We discuss children's temperamental vulnerability versus differential susceptibility to the environment.