Children infected with human immunodeficiency virus (HIV) often lose their vaccine-induced antibody to measles virus. Before highly active antiretroviral therapy (HAART), an additional immunization against measles infrequently resulted in protective antibodies. The antibody response to an additional measles-mumps-rubella (MMR) vaccination was compared in 28 HIV-infected children who lacked protective antibody to measles virus and were undergoing HAART or non-HAART regimens. Serostatus was measured by automated enzyme-linked immunoassay. Nine (64.3%) of 14 children undergoing HAART, compared with 3 (21.4%) of 14 in the non-HAART group, had antibody to measles virus after the additional vaccination with MMR (P=.027). The groups showed no significant difference in CD4 cell values. Ten of 14 HAART patients had undetectable levels of HIV. The mean HIV load for the HAART group was 27,700 copies/mL (median, <400 copies/mL); for the non-HAART group, it was 86,000 copies/mL (median, 9000 copies/mL). Thus, HAART improves the response to an additional MMR vaccination, which is consistent with immune system reconstitution.