The ability to accurately measure viral RNA in the plasma (1-3) and intracellular (4-7) compartments of HIV-1-infected persons has led to a dramatic improvement in the understanding of the natural history of HIV-1 and AIDS. A number of recent studies have convincingly demonstrated that high levels of viral replication occur at all stages of disease (8-10), and that changes in viral RNA load are predictive of disease outcome (11,12), and response to therapy (13,14). These findings, combined with the introduction of potent new antivirals (15,16), have stimulated a growing interest in viral load monitoring, both as a function of disease status, and as a predictor of disease progression and therapeutic efficacy.