INTRODUCTION: As of May 2003, 17% of all reported AIDS cases in Puerto Rico had occurred among those 20-29 years of age. These individuals were likely initially infected with HIV in adolescence. The objectives of this study are to describe and compare the prevalence of the sociodemographic, risk-behavior, and substance-use profile among patients infected with HIV during their adolescence (early) and compare them with patients infected at an older age (non-early). METHODS: This is a cross-sectional study of 3151 HIV/AIDS patients admitted to the retrovirus research center of our institution between 1992 and 2002. The variables we studied include the presence or absence of early infection, sociodemographic variables, risk-behavior variables, and substance-use variables. An early-infected patient was defined as a patient with HIV/AIDS that reported his or her first positive HIV test result before the age of 21 years. Descriptive and differential analyses were performed. RESULTS: Five percent of our patients were early-infected (157/3151). A significantly higher proportion (P < or = .05) among the early-infected patients as compared to the older group was females, required inpatient hospital care at the time of study entry, and had less than a 12th-grade education. In the interview a significantly higher proportion had antisocial behavior, had been in prison at some point in their life, and had used crack-cocaine. CONCLUSIONS: The early HIV-infected patients showed a different sociodemographic, risk-behavior, and substance-use profile. Knowledge of the specific characteristics of early HIV-infected patients could be used to develop primary prevention programs directed toward reducing HIV infection among young Puerto Ricans.