This article discusses the results of a study of a group of 46 aggressive boys, aged 8 to 9 years of age, who participated in a prevention program aimed at reducing their externalizing problems in the short term and at preventing delinquency in the long term. Outcome measures were collected during the 3-year period following the prevention program. At that time, the boys were 10, 11, and 12 years of age. Outcome measures included teacher ratings of aggressiveness and self- reports of delinquent behaviors. The boys' friends were rated on disruptive behaviors by their classmates. The experimental boys' outcome measures were compared to the measures obtained from 58 aggressive boys in a control group. The disruptive ratings of the experimental and control subjects' best friends also were compared. Differences in teacher-rated aggressiveness between the two groups increased from one year to the next for the 3- year follow-up period. Similarly, the friends' disruptive scores differed increasingly between the two groups over the 3-year period. The effect of the prevention program on the subjects' friendships and the mediating impact of friends with regard to social adjustment were stressed. Results do not clearly support the notion that aggressive behavior is a necessary precursor of delinquency since other behaviors may have been manipulated through the prevention program. Some evidence suggests that family functioning, academic, and social-cognitive skills had been affected. Suggestions for future research are offered. Tables, references