Objective: To examine potential hematologic and immunologic markers for healthy adolescents and for adolescents infected with HIV. Design: The REACH Project (Reaching for Excellence in Adolescent Care and Health) of the Adolescent Medicine HIV/AIDS Research Network (AMHARN) recruits HIV-infected and high-risk HIV-uninfected adolescents, aged at least 13 but less than 19 years. The study evaluates biomedical and behavioral features of HIV infection as observed while under medical care for HIV infection and adolescent health. Methods: Blood samples were collected from HIV-infected and HIV-uninfected subjects at 16 clinical sites. Cell phenotypes were determined using standard single, dual or three-color flow cytometry. Results: This report includes data at enrollment for 94 HIV-positive adolescents who had never received antiretroviral therapy (ART) (mean age, 17.4 ± 1.0 years for males and 16.5 ± 1.3 years for females) and 149 HIV-negative adolescents (mean age, 16.7 ± 1.2 years for males and 16.6 ± 1.2 years for females); this is the antiretroviral therapy-naive subset drawn from 294 HIV-positive and 149 HIV-negative adolescents enrolled in the REACH Cohort. The total leukocyte count was significantly reduced in the HIV-positive females in comparison with the HIV-negative females (P < 0.001). There was a reduction in natural killer cells (P < 0.05) in HIV-positive females (mean, 140.6 ± 104.2 x 106 cells/l) in comparison with HIV-negative females (184.3 ± 142.5 x 106 cells/l), whereas no differences were found between the two groups of males. The reduction in the total CD4 cell count in HIV-positive males and females in comparison with the HIV-negative subjects was the consequence of a decrease in both the naive CD4 and memory CD4 components. There was a striking increase in the mean number of CD8 memory cells in HIV-positive compared with HIV-negative adolescents, and a corresponding increase in the percentage of these cells. In contrast, naive CD8 cells were present in increased numbers but their percentage was decreased. Conclusions: These studies of adolescents provide normative data for high-risk healthy adolescents as well as baseline immunologic data for a cohort of ART-naive HIV-positive adolescents. This comparison suggests that this untreated, recently infected group had relatively intact immunologic parameters.