This paper takes up the concern that sexual health programs targeting adolescents may actually increase HIV risk among youth by reinforcing dominant versions of masculinity that portray males as sexually irresponsible and unconcerned about their health. If a key aim in HIV prevention education is a renegotiation of high-risk behavioral norms, an important consideration is the ways young people resist stereotypical gender norms that can lead to risky sexual practices. From this perspective, opening up spaces for the expression of counter-hegemonic masculinities may be an important health prevention strategy. In a study conducted in three urban Toronto high schools, we explore the ways students in mixed-sex groups supported or challenged dominant discourses of masculinity expressed through three themes: notions of male sexuality as unrestrained and unrestrainable; narrow definitions of sex; and concepts of "risk" and resistance to condom use. We argue that designing HIV prevention programs that "begin" with the exploration of alternative masculinities may be one way to fashion a framework for gender relations that can offer youth more effective prevention strategies. (Contains 3 notes.)