The polymerase chain reaction assay, branched DNA assay and nucleic acid sequence-based amplification assay quantitate human immunodeficiency virus (HIV) RNA levels. Plasma viral load (PVL) testing has become a cornerstone of HIV disease management. Initiation of antiretroviral drug therapy is usually recommended when the PVL is 10,000 to 30,000 copies per mL or when CD4+ T-lymphocyte counts are less than 350 to 500 per mm3 (0.35 to 0.50 x 10(9) per L). PVL levels usually show a 1- to 2-log reduction within four to six weeks after therapy is started. The goal is no detectable virus in 16 to 24 weeks. Periodic monitoring of PVL is important to promptly identify treatment failure. When feasible, the same assay should be used for serial PVL testing in the individual patient. At least two PVL measurements usually should be performed before antiretroviral drug therapy is initiated or changed. PVL testing may be helpful in the rare instance of indeterminate HIV antibody testing, especially in a patient with recent infection.