Objective: To assess whether a multimedia program can affect counseling behavior related to one of the strongest risk factors for violence later in life, persistent early childhood aggression. Method: The design was a controlled trial with unobtrusive measurement in a clinic setting. A researcher, pretending to be the mother of a 2 1/2 year old boy, called 19 pediatric residents during clinic hours and requested advice on how to manage her child's persistently hurtful behavior. The intervention was a 40-min lecture focusing on a multimedia program, "Play Nicely", which teaches accepted strategies for managing aggression in young children ages 1-7 years. Residents' responses were blindly assessed to determine the treatment effect of the intervention. Results: Compared with the control group (C), residents in the intervention (I) group were more likely to recommend setting the rule (I: 100% vs. C: 31%, p = .01), redirecting (I: 83% vs. C: 8%, p = 0.003), promoting empathy (I: 50% vs. C: 0%, p = 0.02), and more likely to discourage the use of physical punishment (I: 83% vs. C: 31%). These are the primary strategies encouraged by the intervention. The magnitude of the effect size was very large for each of these three strategies, ranging from d = 1.1 to 2.3. Conclusions: A brief intervention can improve the counseling behavior of primary care physicians regarding persistent childhood aggression. The findings have implications for child abuse prevention, violence prevention, medical education, and how to improve anticipatory guidance within primary care. (Contains 3 tables.)