Increasing access to sterile syringes and new drug preparation materials is an effective means of reducing HIV transmission among injection drug users (IDUs), and a fundamental component of harm reduction ideology. The purpose of this study is to examine changes during a three-year period in syringe acquisition by street-recruited Puerto Rican IDUs characterized by frequent drug injection and high HIV seroprevalence. At baseline (1998-1999) and 36-month follow-up, 103 IDUs recruited in East Harlem, New York (NY), and 135 from Bayamón, Puerto Rico (PR) were surveyed about syringe sources and HIV risk behaviors in the prior 30 days. A majority of participants in both sites were male (NY 78.6%, PR 84.4%), were born in Puerto Rico (NY 59.2%, PR 87.4%), and had not completed high school (NY 56.3%, PR 51.9%). Compared to PR IDUs at follow-up, NY IDUs injected less (3.4 vs. 7.0 times/day, p < .001), and re-used syringes less (3.1 vs. 8.0 times, p < .001). Between baseline and follow-up, in NY the proportion of syringes from syringe exchange programs (SEPs) increased from 54.2% to 72.9% (p = .001); syringes from pharmacies did not increase significantly (0.2% to 2.5%, p = .095). In PR, the proportions of syringes from major sources did not change significantly: private sellers (50.9% to 50.9%, p = .996); pharmacies (18.6% to 19.0%, p = .867); SEP (12.8% to 14.4%, p = .585). The study indicates that NY SEPs became more dominant, while NY pharmacies remained a minor source even though a law enacted in 2001 legalized syringe purchases without prescription. Private sellers in PR remained the dominant and most expensive source. The only source of free syringes, the SEP, permitted more syringes to be exchanged but the increase was not statistically significant. Implications for syringe exchange and distribution programs are discussed.