The authors assessed whether low executive cognitive functioning (ECF) and a difficult temperament are related to aggressive and nonaggressive forms of antisocial behavior (ASB) in 249, 14–18-year-old, conduct-disordered females and controls. ECF was measured using neuropsychological tests; temperament was measured using the Dimensions of Temperament Survey—Revised; and ASB was assessed using psychiatric symptom counts for conduct disorder. The conduct-disordered females exhibited lower ECF capacity and a greater difficult temperament compared with the controls. The combined influence of low ECF and difficult temperament was significantly related to both forms of ASB. In comparison with low ECF, difficult temperament was more strongly related to nonaggressive ASB, whereas in comparison with difficult temperament, low ECF was more strongly related to aggressive ASB. Last, ECF mediated the relation between difficult temperament and aggressive ASB.