Limited data suggest that HIV-infected pregnant women develop postpartum viral rebound. However, viral load changes in the postpartum period have not been adequately characterized. Changes in adherence to antiretroviral therapy, pregnancy-related changes in pharmacokinetics of antiretroviral medications, and decline in immune competence are mechanisms by which postpartum viral load rebound may occur. This study is designed to characterize the incidence and magnitude of postpartum viral rebound during the initial 24 weeks postpartum and to explore the mechanisms and consequences of viral rebound.