Recommendations for the age at vaccination should balance the need to minimize the risk of morbidity and mortality with the benefit of achieving the highest seroconversion rates. Immunizing a more intact immune system at an earlier stage of HIV infection may in turn achieve better and long-lasting measles protection. This study will help define a more effective measles vaccine regimen for children diagnosed with HIV infection and will provide greater insight into the functional status of the HIV-infected children's humoral immune system.