This study replicated and extended the design and outcome measures of several small studies. In these studies, juveniles at high risk for violence and delinquency showed decreased violence and positive changes in psychological risk factors after being required to take a school-linked course in traditional martial arts. In the present study, 60 boys in a large urban middle school were required to take a traditional martial arts course in their school. They were paired on problematic behavior profiles and assigned to a treatment group or to a wait-list control group. Thirty classes, three per week (45 minutes each), were taught by a master of Koga Ha Kosho Shorei Ryu Kempo and his assistant (neither was a public school teacher). Results are reported here for 14 variables from the following measures: four teacher rating scales from the Sutter-Eyberg Student Behavior Inventory, five self-report scales of the Piers-Harris Self-Concept Scale, four computerized measures of attentional self-control from the Intermediate Visual and Auditory Continuous Performance Test, and a count of permanent expulsions from school. The treatment students improved over baseline on 12 variables, while the controls improved on 5 by small amounts and deteriorated from baseline on 8, including teacher-rated violence. There were significant differences between the groups on self-reported happiness and schoolwork and on one measure of attention. After controls took the course, their scores resembled the postcourse scores of the treatment group. Importantly, the control group's increase in teacher-rated violence was reversed. Both groups were then pooled to compare baseline and postcourse teacher ratings. Their scores improved significantly in the areas of resistance to rules, impulsiveness, and inappropriate social behavior. There was also improvement in regard to violence, but the change in scores was not statistically significant. Follow-up on teachers' ratings showed that improvement remained, and in some cases increased, four months after completion of the course. Interestingly, all 6 permanent expulsions were among the control group students who had not yet taken, or had only begun taking, the martial arts course.