Predictors and concurrent correlates of childhood-onset and adolescent-onset antisocial behavior were studied in a sample of 165 boys and 151 girls, followed from age 6 to age 15. An integrated general growth mixture model was used to determine the number and shape of developmental trajectories of antisocial behavior exhibited by boys and girls. Associations of these trajectories with trajectories of attention-deficit/hyperactivity (ADH) problems and deviant peer affiliation were examined. A childhood-onset, an adolescent-onset, and a low antisocial behavior trajectory were identified. A minority of the sample (11%), mostly males, followed the childhood-onset trajectory. This trajectory was predicted by prior membership in the high ADH trajectory in childhood. The adolescent-onset trajectory (46%) was associated with increases in friends' antisocial behavior but not with ADH problems. Most females (60%) followed the low antisocial trajectory. A substantial proportion of females, however, followed the childhood (5%) and adolescent-onset trajectories (35%). The male-female ratios in the childhood and adolescent-onset trajectories were similar. The results largely supported theories that distinguish between childhood and adolescent onsets of antisocial behavior, but they did not suggest that boys and girls differ in the age of onset of antisocial behavior. © 2007 Cambridge University Press.