The relationship between exposure to aggression in the media and children's aggressive behavior is well documented. However, few potential solutions have been evaluated. To assess the effects of reducing television, videotape, and video game use on aggressive behavior and perceptions of a mean and scary world. Randomized, controlled, school-based trial. Two sociodemographically and scholastically matched public elementary schools in San Jose, Calif. Third- and fourth-grade students (mean age, 8.9 years) and their parents or guardians. Children in one elementary school received an 18-lesson, 6-month classroom curriculum to reduce television, videotape, and video game use. In September (preintervention) and April (postintervention) of a single school year, children rated their peers' aggressive behavior and reported their perceptions of the world as a mean and scary place. A 60% random sample of children were observed for physical and verbal aggression on the playground. Parents were interviewed by telephone and reported aggressive and delinquent behaviors on the child behavior checklist. The primary outcome measure was peer ratings of aggressive behavior. Compared with controls, children in the intervention group had statistically significant decreases in peer ratings of aggression (adjusted mean difference, -2.4%; 95% confidence interval [CI], -4.6 to -0.2; P =.03) and observed verbal aggression (adjusted mean difference, -0.10 act per minute per child; 95% CI, -0.18 to -0.03; P =.01). Differences in observed physical aggression, parent reports of aggressive behavior, and perceptions of a mean and scary world were not statistically significant but favored the intervention group. An intervention to reduce television, videotape, and video game use decreases aggressive behavior in elementary schoolchildren. These findings support the causal influences of these media on aggression and the potential benefits of reducing children's media use.