We report results of a community study of syringe exchange that compared the HIV risk behavior of exchange clients with that of nonclients. A prospective cohort of 259 untreated injecting drug users (IDUs) was followed a mean of 10.7 months after baseline. In assessing whether HIV risk behavior at follow-up was associated with study participants' follow-up use of the exchange, we controlled their baseline risk behavior and exchange use as markers of risk-taking tendency in addition to other potential confounders. We also examined whether there was a differential benefit of exchange use for IDUs with versus those without access to other sources of syringes. Both univariate and multivariate analyses revealed a more than twofold decreased odds of HIV risk behavior associated with use of the exchange. In a second multivariate analysis, which examined the interaction of exchange use with access to other sources of syringes, the odds of HIV risk behavior were decreased more than sixfold for IDUs without other sources. We conclude that use of the exchange had a substantial protective effect against HIV risk behavior and may have been especially critical for IDUs without other sources of syringes.