Purpose: The entire Transtheoretical Model, consisting of stages of change, processes of change, self-efficacy, Adolescents and decisional balance, was tested for its applicability to understanding exercise behavior in an adolescent population. Methods: Students (n = 819) from five community high schools (Grades 9-12) completed self-administered questionnaires, all of which were adapted from previous literature. Stage of exercise behavior change served as the dependent variable. Results: Distribution of the sample across the stages was: precontemplation, n = 17 (2.1%); contemplation, n = 34 (4.2%); preparation, n = 235 (28.7%); action, n = 129 (15.7%); and maintenance, n = 404 (49.3%). Univariate analyses of variance (p < 0.0038) followed up with Tukey's post-hoc analyses revealed that all of the constructs differed significantly across stages. Profile analysis indicated that pros and cons intersected in the action stage using t scores; however, when analyzing raw scores, this intersection was found to be in the precontemplation stage. Discussion: Preliminary evidence was found for the applicability of the Transtheoretical Model to adolescents. The low rate of adolescents in the precontemplation and contemplation stages was not deemed a major limitation, owing to the purpose of the investigation. (C) Society for Adolescent Medicine, 1998