Background: Physical aggression is an important issue in North American populations. The importance of students' social environments in the occurrence of physical aggression requires focused study. In this study, reports of physical aggression were examined in relation to social environment factors among national samples of students from Canada and the United States. Methods: Students in grades 6-10 from the United States (n = 14,049) and Canada (n = 7058) who had participated in the "Health Behaviour in School-aged Children Survey" (HBSC) were studied. Rates of students' physical aggression were compared between the 2 countries. School, family, socioeconomic, and peer-related factors were considered as potential risk factors. A simple social environment risk score was developed using the US data and was subsequently tested in the Canadian sample. Results: Risks for physical aggression were consistently higher among United States versus Canadian students, but the magnitude of these differences was modest. The relative odds of physical aggression increased with reported environmental risk. To illustrate, US boys in grades 6-8 reporting the highest social environment risk score (5+) experienced a relative odds of physical aggression 4.02 (95% CI 2.7-5.9) times higher than those reporting the lowest score (adjusted OR for risk scores 0 through 5+ was 1.00, 1.19, 2.10, 2.01, 3.71, and 4.02, respectively, p[subscript trend] less than 0.001). Conclusions: Unexpectedly, rates of physical aggression and associations between social environments and students' aggression were remarkably similar in Canada and the United States. Family, peer, and school social environments serve as risk or protective factors, with significant cumulative impact on physical aggression in both countries. Given the observed high rates and the many negative effects of aggression on long-term health, school policies aimed at the reduction of such behavior remain a clear priority. (Contains 1 figure and 3 tables.)