A longitudinal sample of 916 adolescents was examined to assess the extent to which the perceived smoking prevalence of adults' or peers' smoking was related to cigarette smoking. Questionnaires were distributed to junior high school students in Grade 7 and again in Grade 9. Prevalence of perceived peers' smoking and prevalence of perceived adults' smoking were significantly related to cigarette smoking both cross-sectionally and longitudinally. Adolescents who believed that half or more than half of all adults or peers smoked cigarettes showed the most smoking involvement, and those who believed that fewer than half of adults or peers smoked were least involved. These findings provide further evidence that adolescent normative expectations about cigarette smoking are an important determinant of smoking initiation.