Background. The transition from adolescence to adulthood, referred to as emerging adulthood, is accompanied by an increase in risky sexual activity and injection drug use, two behaviors that are the main routes of transmission of sexually transmitted infections and HIV (STI/HIV). Given that adolescents and emerging adults are disproportionately affected by STI/HIV, understanding adolescent predictors of risk behaviors for STI/HIV in emerging adulthood is important to mitigate STI/HIV risk among youth. Three dimensions of friendships (friendship quantity, quality and the friends' risk behaviors) are particularly useful in predicting adolescents' and young adults' STI/HIV risk behaviors. These friendship dimensions have often been considered separately. But recent research suggests that they interact. This study therefore combined developmental and public health perspectives to examine whether dimensions of adolescent friendships interact to influence emerging adults' STI/HIV risk behaviors. Gender and racial differences were investigated.

Methods. Analyses were based on 1,154 respondents, aged 12-18 years at baseline, who participated in Waves I and III of the National Longitudinal Study of Adolescent Health. The STI/HIV risk behaviors scale was created as a composite measure of the sexual activities and drug use behaviors that directly influence STI/HIV transmission. Independent variables included: number of friends (friendship quantity), emotional and behavioral closeness to friends (friendship quality), and friends' STI/HIV risk behaviors. Multiple linear regression models were used to examine the study's aims.

Results. In emerging adulthood, the mean number of STI/HIV risk behaviors respondents had engaged in was 3.26 (range: 0-7). No interaction effects were observed between friends' STI/HIV risk behaviors and: number of friends, emotional and behavioral closeness to friends. There was a weak positive relationship between STI/HIV risk behaviors in adolescence and those in emerging adulthood, and a direct effect of behavioral closeness to friends on STI/HIV risk behaviors in emerging adulthood. Most associations did not vary by gender or race.

Conclusion. Closer friendships in adolescence predicted more STI/HIV risk behaviors in emerging adulthood, regardless of the friends' STI/HIV risk behaviors. Findings are discussed in light of developmental and public health perspectives on the importance of friendships in adolescence.