Sharing of contaminated needles constitutes one of the primary modes of HIV transmission in the U.S. This study examined social and environmental factors as possible correlates of needle sharing in a sample of inner-city drug users in Baltimore, Maryland. Drug users' social context was assessed through an analysis of personal networks, and the environmental context of drug use was assessed through a delineation of injection settings. The 330 respondents participating in an HIV prevention study were administered two surveys an average of 5.2 months apart. An examination of the association of network characteristics at baseline and needle sharing at followup revealed that the factors of higher total network density, larger drug network size, and injecting at friends' residences were positively associated with reports of sharing needles that had been cleaned with bleach. Sharing of needles that had not been disinfected with bleach was positively associated with reports of injecting in semipublic areas (streets, rooftops, parks, cars, public bathrooms, and abandoned buildings). These data support ecological and resource models of needle sharing and suggest the potential utility of network-oriented strategies for reducing needle sharing among injecting drug users.