Objective: To evaluate the effectiveness of a social cognitive intervention program for Dutch aggressive boys and to compare it with a social skills training and a waitlist control group. Method: A randomized, controlled treatment outcome study with 97 aggressive boys (aged 9-13 years) was presented. An 11 session group treatment, a social cognitive intervention program (n = 42) based on Dodge's social information-processing theory, was compared with social skills training (n = 40) and waitlist control group (n = 15). Measures of aggressive behavior, self-control, social cognitive skills, and appropriate social behavior were completed before and after the group treatment and at 1-year follow-up. Results: The outcome of both treatment conditions indicated (1) a significant increase in appropriate social behavior, social cognitive skills, and self-control and (2) a significant decrease in aggressive behavior. There was a significant difference between treatment and no treatment and between the social cognitive intervention program and social skills trainingon various child, parent, and teacher measures. Conclusions: The expectation that focusing on the deficits and distortions in social cognitive processes (social cognitive intervention program) instead of merely focusing on social skills (social skills training) would enhance the effectiveness was supported on child, parent, and teacher measures. At 1-year follow-up, the mean effect sizes of the social cognitive intervention program and social skills training were 0.76 and 0.56, respectively. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(12):1478-1487. Key Words: aggressive behavior; self-control, social information processing, cognitive behavior therapy, social skills training.