Depletion of CD4 cells is a hallmark of progressive human immunodeficiency virus (HIV) disease1 and a powerful predictor of the short-term risk of progression to AIDS.2 Untreated HIV infection leads to AIDS in approximately half of untreated patients within 10 years,3 but the rate at which AIDS occurs in each individual varies greatly. Both CD4 cell count and plasma HIV RNA level discriminate independently between rapid and slow progressors4; thus, the 9-year probability of AIDS ranges from 3.6% among patients with low plasma HIV RNA levels and high CD4 cell counts to 100% among patients at the opposite end of the spectrum.5 In addition to their role as predictors of the clinical outcomes of HIV infection, CD4 cell count and plasma HIV RNA level are commonly used as markers of the success of highly active antiretroviral therapy (HAART). Large clinical trials using currently recommended antiretroviral combinations suggest that over three fourths of individuals starting HAART will achieve a plasma HIV RNA level below the limit of detection of clinically available assays, and most of them, but not all, will experience concomitant CD4 cell increases.6-8