Anger can be defined as an emotion consisting of feelings of variable intensity, from mild irritation or annoyance to intense fury and rage. Borderline personality disorder (BPD) is characterized by impulsivity and instability of interpersonal relationships, of self-image, and of negative affects. Borderline personality and trait anger are often observed together. The present study examined the extent to which a genetic association explains the covariation between a trait measure of borderline personality and trait anger. To this end, self-report data of 5,457 twins and 1,487 of their siblings registered with the Netherlands Twin Register and the East Flanders Prospective Twin Survey were analyzed using genetic structural equation modeling. A significant phenotypic correlation was observed between the two traits (rP=.52). This correlation was explained by genetic (54%) and by environmental influences (46%). A shared genetic risk factor is thus one of the explanations for the covariation of borderline personality and trait anger. © 2011 American Psychological Association.