The REACH Project (Reaching for Excellence in Adolescent Care and Health) of the Adolescent Medicine HIV/AIDS Research Network was designed as a study of an adolescent cohort composed of HIV-1-infected and -uninfected subjects. The goal of the analysis presented was to examine the relationship of CD4+ T cell counts and HIV-1 plasma vital loads in adolescents. The CD4+ T cell counts of 84 HIV+ subjects who were 13 to 19 years of age were measured at the clinical sites, using ACTG standardized techniques. HIV-1 vital loads in frozen plasma were determined by the NASBA/NucliSens assay at a central laboratory. Past and current treatment with antiretroviral drugs was determined by medical record abstraction and interview data. The slope of the line generated by regressing log10 HIV-1 RNA (copies/ml versus CD4+ T cell counts of REACH subjects who are antiretroviral drug naive was negative and significantly different than zero. A negative association has also been reported for antiretroviral drug-naive, adult males in the Pittsburgh Men's Study, a component of MACS (Pitt-MACS) (Mellors J, et al.: Science 1996;272:1167). These data show that in adolescents, as in adults, HIV-1 RNA concentrations are correlated with corresponding absolute CD4+ T cell count. The slopes of the lines generated with data from each cohort were different (p = 0.003). In addition to age, there are sex and racial differences in the makeup of the two cohorts. Any or all of these differences may affect the slopes of the lines.