In this study, the social network characteristics of 102 urban adolescents in brief substance abuse treatment are described and analyzed longitudinally to examine risk and protective mechanisms. The treatment intervention had one session devoted to social support and networks. Social networks were conceptualized and measured along two dimensions (a risk and a protective dimension) with four categories within each dimension. Results indicate a reduction in risk and an increase in protective characteristics at 12-month follow-up and point to specific characteristics that increase risk for and protection against substance use and psychiatric problems. These findings support a social network approach in understanding adolescent health behaviors. (Contains 4 tables.)