The relationship between morningness/eveningness (M/E) and morning-to-afternoon cortisol ratio, pubertal timing, and antisocial behavior was examined in 111 girls and boys ages 8 to 13 years. Cortisol levels showed a significant increase after awakening and declined thereafter (p less than .05). Eveningness was related to a composite measure of antisocial behavior and rule-breaking and attention behavior problems and conduct disorder (CD) symptoms in boys and relational aggression in girls. In boys only, lower a.m. to p.m. cortisol ratio, indicating less circadian decrease in cortisol, was related to attention problems. Early pubertal timing was associated with boys' rule-breaking and attention behavior problems and CD symptoms and girls' relational aggression. The findings indicate that evening activity preference; extreme a.m. to p.m. cortisol ratios, in one case; and early pubertal timing were associated with antisocial behavior even in young adolescents, but the findings were stronger for boys than for girls.