Human immunodeficiency virus (HIV)-infected patients (n = 153) failing antiretroviral therapy after exposure to compounds from all three drug families were monitored for 6 months after beginning a rescue intervention program including tenofovir (TDF). At 3 months, levels of HIV RNA in plasma dropped by a mean of 0.9 log(10) and the mean CD4 count increased by 52 cells/ micro l. At 6 months, HIV RNA levels had dropped by a mean of 1.06 log(10) and the mean CD4 count had increased by 49 cells/ micro l. Only five (3.7%) patients discontinued TDF use due to adverse events. In the multivariate analysis, the presence of M41L and/or L210W at baseline was the only viral determinant of a lower response to TDF.