A prospective study of long-term clinical outcomes in 180 HIV-infected patients followed for more than 10 years showed a strong prognostic relationship between the viral load and the likelihood of developing AIDS. Within five years of infection with HIV, 62% of patients who were in the highest quartile of plasma viral levels developed AIDS, in contrast to only 8% of patients in the lowest quartile. These data helped to establish recognition of the viral load as a highly sensitive predictor of HIV disease progression independent of CD4 cell count.