The ability of memory T cells to mount a recall response plays a key role in the ability of vaccinated animals to contain viral challenge. In this study, we intensively monitored the expansion of SIV Gag-specific CD8+ T cells in peripheral blood and tissues of rhesus macaques vaccinated with the attenuated strain SIVmac239Delta3 and challenged with the pathogenic viruses SIVmac239 or SIVsmE660. Although all vaccinated animals were infected with challenge virus, peak levels of plasma viremia in vaccinees were decreased by 1.5 to 2 logs as compared with naive controls. Decreased levels of plasma viremia in vaccinated animals were evident as early as 7 days post-challenge, well before the expansion of SIV-specific CD8+ T cells. Expansion of SIV-specific CD8+ T cells was not observed in peripheral blood or tissues until at least 14 days after infection and did not occur in most animals until after the initial peak of viral replication. The observation that expansion of SIV-specific CD8+ T cells is delayed until 7 days or more after initial detection of viremia highlights fundamental limitations in the ability of lentivirus-specific CD8+ T cells to mediate protection against challenge.