PURPOSE: This study examined the associations of free testosterone and family environment with delinquent and aggressive behaviors among adolescent boys and girls with elevated low-density lipoprotein (LDL)-cholesterol levels. METHODS: Participants were 164 boys and 180 girls 11-14 years of age. The female parent provided ratings of family cohesion and of child aggressive and delinquent behaviors. Tanner ratings of pubertal development were obtained during physical examination, and a blood sample was drawn for assessment of serum levels of free testosterone. RESULTS: Hierarchical regression analyses revealed significant two-way interactions of free testosterone and family cohesion on delinquent behaviors among adolescent boys and girls. Specifically, under conditions of low family cohesion, free testosterone was positively associated with delinquent behaviors among boys, whereas in families with high cohesion no association between free testosterone and delinquent behavior was observed. In contrast, free testosterone was negatively associated with delinquent behaviors among adolescent girls in low-cohesion families. For girls, family cohesion was negatively associated with aggressive behaviors; for boys, LDL-C was positively associated with aggressive behaviors. CONCLUSIONS: Child gender and family environment factors appear to modify the associations between free testosterone and delinquent behaviors in adolescent boys and girls.