Patterns of low hypothalamus-pituitary-adrenal (HPA) activity have been observed in antisocial groups. As conflicting results have been reported in children and adolescents, the aim of this study was to further investigate HPA activity in antisocial behavior by studying the relationship between the diurnal cortisol cycle, as well as the cortisol awakening response (CAR), and antisocial behavior in male adolescents. The diurnal cortisol cycle and the CAR during the first hour after awakening were compared between 12- to 14-year-old boys who attended a delinquency diversion program (DP), with and without a disruptive behavior disorder (DBD) (respectively DP+; n=24 and DP-; n=65), and matched normal controls (NC; n=32). The DP+ group, but not the DP- group, showed a significantly slower decrease of cortisol during the diurnal cycle than the NC group. Furthermore, the DP+ group had significantly lower cortisol levels in the first hour after awakening as compared with the NC group. The results indicate altered HPA activity in delinquent boys with a DBD. Etiological mechanisms, directions for future research, and clinical implications are discussed.