Interventions are increasingly sensitive to the role of social networks and cultural dynamics on behavioral change, reflecting a shift in the focus of prevention strategies from individuals to communities. This study addresses the contextual issue of relationship power in young injection drug users (IDU), an at-risk population for HIV, hepatitis C virus (HCV) and other blood borne pathogens. The overall objective of this mixed-method study is to (1) develop a grounded theory of relationship power associated with risky injection behaviors and define high risk injecting relationships, and (2) develop a theoretically driven and validated measurement scale for relationship power within injecting relationships. Utilizing existing infrastructure within the UFO Study, we will first describe the multifaceted power issues relevant to HIV risk behaviors within injecting partnerships among young IDU. Through 30 in-depth interviews and 2 focus groups we will define different injecting relationships and identify key domains that comprise power imbalances within these injecting relationships to form the basis of scale items. Next, we will fill in an important methodological gap by developing and validating a scale to measure relationship power within 100 injecting partnerships. The Mplus program will be used to conduct factor analyses. The SAS program will be used to conduct the preliminary, reliability, and validity analyses. Unlike for sexual relationships, there have been no scales developed to assess relationship power in injecting partnerships. The proposed project will provide necessary findings for subsequent submission of a K01 award to examine the association between relationship power and HIV risk in injecting relationships internationally and to develop a relationship-based pilot intervention for young IDU.