We evaluated the virological outcome of lopinavir/ritonavir (LPV/RTV) in 224 HIV-1-infected and protease inhibitor (PI)-experienced patients showing virological failure to a highly active antiretroviral therapy (HAART) regimen and followed up for at least 3 months. At baseline, the median level of plasma viraemia was 4.61 log10 copies/ml (range 3-6.48) and the median CD4 cell count was 219 cells/mm3 (range 1-836). During a median follow-up of 272 days (range 92-635), we observed an increase in the number of CD4 cells (P=0.02) and a dramatic decrease in plasma viraemia levels (P=0.0001), which became undetectable in 122 patients (54.5%). The closely related predictive factors were baseline plasma viraemia levels and the number of mutations known to reduce susceptibility to LPV/RTV. Thirty-one patients (13.8%) discontinued LPV/RTV during the follow-up, and one AIDS event and three deaths were recorded. Of the 134 patients (59.8%) who underwent a baseline genotype resistance test, 22 (16.4%) had > or = 6 mutations known to reduce LPV/RTV susceptibility; plasma viraemia became undetectable in 76 patients (56.7%), only five of whom harboured > or = 6 mutations at baseline (P=0.0001). The independent predictive factors related to virological success were plasma viraemia levels and the number of mutations reducing susceptibility to LPV/RTV at baseline; each additional log10 copies/ml of HIV RNA reduced the probability of virological success by 34.0% and each extra mutation by 14.5%.