Since substantial research has shown that a physically active lifestyle is a fundamental component of health, there exists a need to investigate why the majority of individuals do not adhere to an exercise program on a long-term basis. The purpose of this study was to examine the degree to which previous adherence, physical fitness, behavioral intervention, and exercise self-efficacy predict exercise adherence. There were 52 male and 44 female participants in this study ranging in age from 19 to 72 years. All subjects completed the same written and physical evaluations and participated in the same fitness program. Subjects were divided into three groups (experimental: n=34, comparison: n=33, and control: n=29). The experimental group received behavioral intervention aimed at increasing exercise adherence. The comparison group received an intervention which was not aimed at influencing exercise adherence. The control group did not receive any additional intervention. Exercise adherence was determined from self-report of exercise activity for 24 weeks . The behavioral intervention took place during weeks 9 through 16. The 24 weeks were divided into 3 adherence periods: "before adherence" (weeks 1-8), "during adherence" (weeks 9-16), and "after adherence" (weeks 17-24). Pre- and post-treatment exercise self-efficacy was determined from subjects' completion of an Exercise Self-Efficacy Scale. Pre- and post-testing physical fitness assessment included body composition and aerobic capacity measures. The investigator assisted the experimental group participants in identifying realistic goals. These participants met biweekly during the intervention period with the investigator to discuss problems and adjust workouts and goals to adapt to individual progress and personal needs. The results of this study support research studies which have found that previous exercise adherence is highly predictive of continued adherence. Based on a very stringent definition of adherence, on the average, the subjects were unable or unwilling to perform aerobic exercise the three times per week necessary to substantially improve cardiovascular fitness. This finding is consistent with the majority of previous adherence studies. According to the qualitative analyses, improved physical fitness was the primary reason why subjects adhered to aerobic exercise, and a loss of fitness was a strong motivator for participants to begin exercising again after a temporary lapse .