The rate of virological failure was assessed in 819 patients followed up by the private sector of Douala, the economic capital of Cameroon, and treated according to the World Health Organization (WHO) recommendations. In addition, genotypic resistance testing was carried out in the subgroup of 75 selected patients representative of the 254 patients in virological and/or immunological failure receiving a first-line (83%) or second-line (17%) regimen. Overall, 36% of patients treated by antiretroviral drugs (ARV) were in virological failure, as assessed by plasma viral load above 3.7 log(10) copies/ml under treatment for more than 6 months. According to the immunological status, 17% of patients showed a CD4 T cell count under 200 cells/mm(3) and 37% under 350 cells/mm(3), indicating either ongoing immunorestoration or immunological failure under treatment. Twenty percent of patients in virological failure showed wild-type viruses susceptible to all ARV, likely indicating poor adherence. However, 80% of them displayed plasma virus resistant at least to one ARV drug, mostly to the nucleoside reverse transcriptase inhibitors (NRTIs) class (80%), followed by the non-NRTI class (76%) and the protease inhibitor class (19%), thus reflecting the therapeutic usage of ARV drugs in Cameroon as recommended by the WHO. Whereas the second-line regimen proposed by the 2009 WHO recommendations could be effective in more than 75% of patients in virological failure with resistant viruses, the remaining patients showed a resistance genotypic profile highly predictive of resistance to the usual WHO second-line regimen, including in some patients complex genotypic profiles diagnosed only by genotypic resistance tests. In conclusion, our observations highlight the absolute need for improving viral load assessment in resource-limited settings to prevent and/or monitor therapeutic failure.