A 20-week behavioral study was conducted involving adult males (N = 66) in programs of cardiovascular and muscular endurance training. The relationship between exercise adherence and selected psychological and biological variables was examined as was the ability of these variables to discriminate between exercise adherers and dropouts. Results indicated that percent body fat, self-motivation, and body weight discriminated (p < .05) between eventual adherers and dropouts. When combined within a psychobiologic prediction model, these variables accurately classified actual adherers and dropouts in approximately 80% of all cases and accounted for nearly 50% of the variance in adherence behavior. In addition, participants symptomatic with regard to coronary heart disease adhered for a shorter period of time (p < .01) than did those who were asymptomatic. Results did not support theoretical expectations related to the roles of attitude toward physical activity, self-perceptions of physical ability, or locus of health control in the adherence process. These data suggest that the assessment of self-motivation and body composition may substantially enhance the initial diagnosis of the dropout-prone exercise participant and may ultimately assist in adherence facilitation.