Because viral load (amount of HIV RNA in the plasma) is often used as a measure of the effectiveness of new antiretroviral drugs in clinical trials, a washout period, or cessation of current antiretroviral regimens, is commonly required for study entry to allow for a drug-free steady state of viral load prior to initiation of the new drug. However, the kinetics of the viral rebound following drug withdrawal has not been sufficiently studied, and the proper duration of washout is an estimate.