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To examine a parsimonious three-stage model of health behavior change that makes a distinction between non-intenders, intenders, and actors in terms of physical activity. It was hypothesized that intention formation, action planning, and behavior change were at different levels in these three stages, and that these were differentially predicted by self-efficacy, outcome expectancies, and risk perception.Longitudinal analyses with latent multi-group structural equation models using AMOS to test discontinuity patterns in latent means and associations of variables.To examine the discontinuity hypotheses, orthopedic out-patients were assessed at the beginning and the end of their rehabilitation as well as at 6-month follow-up (N=423). Stages, self-efficacy, outcome expectancies, risk perception, intention, action planning, and behavior change were assessed in self-administered questionnaires.In multi-group structural equation models, discontinuity patterns emerged. Differences in latent means, interrelations of social-cognitive predictors, and the amount of explained variance were found between the three stages. Self-efficacy was imperative for all groups of patients. Risk perception was important in intention formation for individuals who had no intention before. The intentional and the actional stages of behavior change were similar in terms of planning.The findings provided support for the usefulness of the three-stage distinction, and the stage-specific prediction of behavior change. To tailor interventions, one should assess stages and address stage-specific needs.