This study assesses the prevalence of injection-related behaviors that may facilitate HIV transmission even when syringes are not directly shared. Subjects who self-reported practicing these "indirect sharing" behaviors are described, the prevalence of these behaviors are compared with the direct sharing of syringes, and variables independently associated with indirect sharing are determined. Injection-related risks were assessed among 585 injection drug users (IDUs) in Denver, Colorado. Respondents were classified into three groups based on their self-reported injection behaviors: IDUs who directly and indirectly share syringes, IDUs who indirectly share only, and IDUs who neither directly or indirectly share. Indirect sharing was twice as prevalent as direct sharing. IDUs who injected heroin or speedballs were less safe in their injection behavior than those who did not. Drug treatment did not protect against injection-related risk behaviors; however, exposure to community HIV prevention efforts was protective. Indirect sharing behaviors are common, yet their link to HIV transmission remains unrecognized by many IDUs. IDUs must be informed about these additional risk behaviors.