To determine the unique contributions of cognitive complexity and pubertal timing to participation in behavioral risk (substance use, sexual activity, school and legal problems) among young adolescents.Cross-sectional with cohort replication.Two cohorts of middle school students in grades eight and nine in a midwestern school district completed a self-report questionnaire in 1987 and 1989. Measures of behavioral and emotional risk, cognitive complexity and pubertal timing relative to peers were included.Complete data were available for 817 and 796 students in each cohort year. Gender, race, level of cognitive complexity and pubertal timing each contributed significantly to the variance in behavioral risk independent of chronological age (P < .0001). Boys, whites, those at lower levels of cognitive complexity and those who began pubertal maturation earlier than peers, were at significantly greater risk. Adolescents who demonstrated higher levels of cognitive complexity and those who began puberty later compared to peers participated in a smaller array of risk behaviors, independent of chronological age.Pediatricians should consider adolescents at lower levels of cognitive complexity (concrete thinking) and those who begin puberty earlier at greater risk for participation in health risk behaviors. Preventive health counseling tailored to the needs of this group may be most beneficial.