OBJECTIVE The present investigation tested the hypothesis that low resting salivary cortisol concentration in preadolescent boys would be associated with aggressive behavior later in adolescence. Second, it tested whether personality traits would mediate this relation. METHODS Resting salivary cortisol concentrations from 314 boys (10-12 years of age) were assayed. When the boys reached 15 to 17 years of age these concentrations were analyzed in the context of personality traits, measured with the Multidimensional Personality Questionnaire, and aggressive behavior, measured with the Youth Self-Report inventory. RESULTS Low cortisol in preadolescence was associated with low harm avoidance, low self-control, and more aggressive behavior 5 years later, during middle adolescence. Cortisol was not related to negative emotionality or any of its factors (including trait aggression). Low self-control was identified as the primary personality mediator of the relation between low cortisol and later aggressive behavior. CONCLUSIONS In adolescent boys, low resting cortisol concentrations appear predictive of clinically important personality factors. Increased aggressive behavior in adolescents with low resting cortisol may be more strongly associated with lack of self-control than with a specifically "aggressive personality."