Using the simian immunodeficiency virus/human immunodeficiency virus (SHIV)-macaque model of AIDS, we had shown in a previous report that a live, nonpathogenic strain of SHIV, further attenuated by deletion of the vpu gene and inoculated orally into adult macaques, had effectively prevented AIDS following vaginal inoculation with pathogenic SHIV(KU). Examination of lymph nodes from the animals at 18 weeks postchallenge had shown that all six animals were persistently infected with challenge virus. We report here on a 2-year follow-up study on the nature of the persistent infections in these animals. DNA of the vaccine virus was present in the lymph nodes at all time points tested, as far as 135 weeks postchallenge. In contrast, the DNA of SHIV(KU) became undetectable in one animal by week 55 and in three others by week 63. These four macaques have remained negative for SHIV(KU) DNA as far as the last time point examined at week 135. Quantification of the total viral DNA concentration in lymph nodes during the observation period showed a steady decline. All animals developed neutralizing antibody and cytotoxic-T-lymphocyte responses to SHIV(KU) that persisted throughout the observation period. Vaccine-like viruses were isolated from two animals, and a SHIV(KU)-like virus was isolated from one of the two macaques that remained positive for SHIV(KU) DNA. There was no evidence of recombination between the vaccine and the challenge viruses. Thus, immunization with the live vaccine not only prevented disease but also contributed to the steady decline in the virus burdens in the animals.