The aims were to identify the correspondence between simultaneous, longitudinal changes in cortisol reactivity and diurnal testosterone and to test the hypothesis that cortisol reactivity and diurnal testosterone interact so as to influence antisocial behavior. Participants were 135 children and young adolescents assessed at 6-month intervals over 1 year. Upon enrollment girls were age 8, 10, or 12 years (N = 69, M = 10.06 years) and boys were age 9, 11, or 13 years (N = 66, M = 10.94 years). Assessments included Tanner staging by a nurse, cortisol reactivity (Trier Social Stress Test for Children), diurnal testosterone, and interviews and questionnaires. Growth models showed that cortisol reactivity and diurnal testosterone basal levels (intercept) and rate of change (slopes) were not related, suggesting different mechanisms of growth. Longitudinal regression analyses assessed cortisol reactivity and diurnal testosterone longitudinally. The interactions of cortisol reactivity and diurnal testosterone showed that when diurnal testosterone was low, boys with low cortisol reactivity were reported to have more behavior problems (i.e., oppositional defiant disorder symptoms and attention problems) than when testosterone was high. In addition, when diurnal testosterone was high, boys with high or moderate cortisol reactivity were significantly higher on total antisocial behavior, attention behavior problems, and oppositional defiant disorder symptoms than when testosterone was low or moderate. The results were similar but less frequent for girls. These findings advance the science of young adolescence by showing the interaction between preexisting sensitivity to stressors and the normative testosterone changes of puberty and antisocial behavior.