This study evaluates the impact of a school-based intervention program on aggressive behavior among junior high school students in Japan. One hundred and four seventh-graders were enrolled in the program and completed Time 1, Time 2, and Time 3 surveys. The program was implemented in two classes between Time 1 and Time 2 surveys (the first treatment group) and in two other classes between Time 2 and Time 3 surveys (the delayed treatment group). The program included four weekly lessons related to problem solving, stress management, and communication. The initial intervention group reported a significant increase in appropriate relationships with classmates. Aggressive behavior significantly decreased from Time 1 to Time 3 in both groups and from Time 2 to Time 3 in the delayed treatment group, but no treatment group effect was noted. Additional modification of the program may be needed to achieve decreases in aggressive behavior. (Contains 3 tables.)