One promising avenue for augmenting the effect of interventions designed to reduce risk behaviors for acquisition of the human immunodeficiency virus (HIV) among adolescents is the use of peer influence through friendship groups. In this study, conducted among 76 groups of African American adolescents, the authors sought to determine whether the similarity of behaviors among group members persists over time and whether the similarity is enhanced through a risk-reduction intervention. Youth within groups were more similar compared to youth between groups with respect to sexual behaviors, expectations, and feelings at the baseline and through 18 months of follow-up. An HIV risk-reduction intervention that increased condom use also increased similarity of condom use among youth within intervention groups at 6 and 18 months postintervention. These data provide empiric support for the hypothesis that delivery of HIV-prevention efforts through groups of friends may be a useful intervention adjunct.