Latent HIV-1 persists in resting memory CD4+ T cells, even in patients receiving highly active antiretroviral therapy (HAART). It has been unclear how stable this latent reservoir is and whether its persistence reflects replenishment by low-level viremia. Here we show that even in treated patients who have had no detectable viremia for as long as 7 years, the reservoir decays so slowly (t(1/2) = 44 months) that eradication is unlikely.