Although previous studies have revealed that preschool-aged children imitate both aggression and prosocial behaviors on screen, there have been few population-based studies designed to reduce aggression in preschool-aged children by modifying what they watch.We devised a media diet intervention wherein parents were assisted in substituting high quality prosocial and educational programming for aggression-laden programming without trying to reduce total screen time. We conducted a randomized controlled trial of 565 parents of preschool-aged children ages 3 to 5 years recruited from community pediatric practices. Outcomes were derived from the Social Competence and Behavior Evaluation at 6 and 12 months.At 6 months, the overall mean Social Competence and Behavior Evaluation score was 2.11 points better (95% confidence interval [CI]: 0.78-3.44) in the intervention group as compared with the controls, and similar effects were observed for the externalizing subscale (0.68 [95% CI: 0.06-1.30]) and the social competence subscale (1.04 [95% CI: 0.34-1.74]). The effect for the internalizing subscale was in a positive direction but was not statistically significant (0.42 [95% CI: -0.14 to 0.99]). Although the effect sizes did not noticeably decay at 12 months, the effect on the externalizing subscale was no longer statistically significant (P = .05). In a stratified analysis of the effect on the overall scores, low-income boys appeared to derive the greatest benefit (6.48 [95% CI: 1.60-11.37]).An intervention to reduce exposure to screen violence and increase exposure to prosocial programming can positively impact child behavior.