Objectives: To describe the demographic characteristics, the risk related practices, and the clinical and immunological features of Puerto Rican HIV injecting drug users (IDU's). Design: Cross-sectional survey of a longitudinal cohort (N=1,500). Methods: The HIV Central Registry at the Universidad Central del Caribe has been recruiting HIV/AIDS patients attending the Immunologic Clinic and the University Hospital Ramon Ruiz Amau since may 1992. 1500 patients have been interviewed and a record including demographic, risk related, clinical and immunologic data has been created for each one of them. Chi square and t-tests were performed to assess the potential significant differences of the IDU's vs the non-IDU's. Results: 57.1% of the HIV/AIDS patients were IDU's. Most of them were males, had a lower education level, with an age fluctuating between 27 to 36 years old and most of them were unemployed. Most IDU's reported to have had heterosexual relations. With regard to the clinical spectrum, Pneumocystis carinii pneumonia (PCP) was the only indicative condition associated with injecting drug use (IDU), as well as bacterial endocarditis, chronic diarrhea, night sweats and idiopathic fever. The spectrum of vulnerabilities of IDU's is larger than that of non-IDU's, with regard to psyche-social, behavioral and clinical features. These vulnerabilities can limit their access to services and render their compliance to treatment more difficult.