The aim of the present study was to explore the treatment effect of tenofovir as implemented in clinical practice. Data are presented on 34 patients. 11 patients had tenofovir added to a stable anti-retroviral treatment (ART) and 23 patients had drugs other than tenofovir. CD4 counts, HIV-RNA levels and genotypic resistance were determined at baseline and after 3 and 6 months. After initiation of tenofovir treatment, a mean decrease in HIV-RNA for all 34 patients was observed (-0.43 log1o copies/ml (+/- 1.22) and -0.49 log10 copies/ml (+/- 1.36) after 3 and 6 months, respectively, (p = 0.045)). However, the effect of tenofovir on HIV-RNA in the group of patients who had tenofovir added to a stable ART was limited, and the decrease in HIV-RNA was significantly higher in patients who had drugs other than tenofovir changed as well (p = 0.004 and p = 0.03 after 3 and 6 months, respectively). After initiation of tenofovir treatment, no significant increases in CD4 count were observed. All new NRTI-associated mutations could be explained by the background treatment. In conclusion, we observed a significant decrease in HIV-RNA only when tenofovir was prescribed, in conjunction with other anti-retroviral drugs, to patients on a failing highly active antiretroviral drug regimen (HAART).