Objective: Basal cortisol and cortisol stress responsivity are valuable biological characteristics of children with disruptive behavior disorder (DBD). In this study, the predictive value of cortisol to outcome of intervention was investigated. Method: Basal cortisol levels and cortisol levels under stress were studied in 22 children with DBD before the start of a psychotherapeutic treatment. The disruptive behavior of the child was assessed before treatment and after cessation (9 months later). Results: Children with DBD with relatively high and low basal cortisol levels differed in the severity of problem behavior at pretreatment, with the low basal cortisol group having more severe problems. During stress, children with DBD showed either increasing or decreasing cortisol values. Although these cortisol responsivity groups were similar in the severity of behavioral problems at pretreatment, the behavioral problems of the group with high cortisol stress responsivity were significantly lower after the intervention than the behavioral problems of the group with low cortisol stress responsivity. Conclusions: In children with DBD, the basal cortisol level was related to the severity of behavioral problems at pretreatment but not to the severity of behavioral problems after treatment. The cortisol response pattern during stress was related to treatment outcome. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(8):1011-1018. Key Words: cortisol, aggression, intervention, disruptive behavior disorder.