We evaluated the efficacy of tenofovir (TDF) - and didanosine (ddI)-containing backbones in HIV-infected experienced subjects. We included in the study 245 subjects who started a TDF/ddI-containing HAART with HIV-RNA > 3 log(10) cp/ml and an available genotypic resistance test at baseline. At baseline, median CD4 counts and HIV-RNA were 278 cell/mmc and 4.32 log(10) cp/ml, respectively. Seventy-four subjects (30.2%) discontinued TDF and/or ddI, 23 of them for drug-related toxicities or intolerance. One-hundred and twenty-six (51.4%) subjects achieved virologic success (HIV-RNA < 50 copies/ml in two consecutive determinations) in a median time of 6.1 months; higher HIV-RNA levels (HR: 0.66, 95% CI: 0.54- 0.79, p < 0.001 for each additional log(10) copies/ml), and the total number of mutations either for PI and NNRTI at baseline (HR: 0.87, 95% CI: 0.81-0.92, p < 0.001 for each additional mutation) were both predictors of virologic success. M184V was marginally associated with virologic success (HR: 1.34, 95% CI: 0.94-1.90, p = 0.10 vs no M184V), whilst the number of TAMs was not associated. One-hundred-thirty-three (54.3%) subjects achieved immunologic success (increase of > or = 100 cells/mm(3) from baseline) in a median time of 7.5 months; immunologic success was associated with HIV-RNA levels at baseline (HR: 0.91, 95% CI: 0.79-0.98, p = 0.04 for each additional log(10) copies/ml), the total number of mutations either for PI or NNRTI (HR: 0.91, 95% CI: 0.85-0.98, p = 0.01 for each additional mutation) and CD4 count at baseline (HR: 1.11, 95% CI: 1.00-1.23, p = 0.05 for each additional 100 cells/mm(3)). Results obtained by the on-treatment analyses were comparable. In our study, HAART containing TDF/ddI seem associated with a virologic and immunologic response, when such regimens are chosen according to a genotypic resistance test.