Objective: The purpose of the study described here is to extend our understanding of needle transfer behaviors among injection drug users (IDUs) by combining both micro and macro variables in a multilevel model. Methods: A sample of 8,404 IDUs interviewed at 18 sites of the National Institute on Drug Abuse (NIDA) Cooperative Agreement for AIDS Community-Based Outreach/Intervention Program (COOP) was used for the study. A multilevel model was employed to deal with intraclass correlation in the sample and to evaluate the impact of micro and macro variables on needle transfer among IDUs. Results: A significant intraclass correlation was detected in the sample. Needle transfer was associated with both the individual characteristics and the features of the social setting in which they were embedded. Some of the micro slopes were fixed, while some varied from site to site, depending on the values of macro variables. The micro intercept and the micro slope of ethnicity vary randomly across social settings, while they are partially determined by macro variables. Conclusion: Considering the intraclass correlation in the data and the effects of contextual features on individual behaviors, multilevel analysis of HIV risk behaviors should be considered when combining data from micro and macro levels (e.g., individual and project site levels).