The current study used social cognitive theory as a framework to investigate self-reported homophobic aggressive behavior at school. Participants included 863 students of 49 classes, enrolled in Grades 9-13 in 10 Italian public high schools. The results from the multilevel mediation model (1-2-1) showed that class-level homophobic attitudes toward gay males mediated the relationship between student observations of peer homophobic aggression and self-reported engagement in homophobic aggression toward schoolmates perceived as gay. However, although student observations of peer aggression toward perceived lesbians predicted self-reported engagement in homophobic aggression toward perceived lesbians, this relationship appeared not to be mediated by class-level homophobic attitudes. Student observations of peer aggression toward perceived lesbians predicted the self-reported engagement in homophobic aggression toward perceived lesbians. It was found that the social cognitive perspective provided considerable insights into homophobic aggression at school. Consistent with this perspective, social and cognitive factors accounted for students' homophobic aggression. (Contains 2 tables and 1 figure.)