OBJECTIVE We evaluate the efficacy and tolerability of ritonavir-boosted dual protease inhibitor as a nucleoside reverse transcriptase inhibitor-sparing regimen in a prospective open-label randomized pilot trial in antiretroviral-naive patients. METHODS Thirty patients received fosamprenavir/atazanavir/ritonavir (Group 1) and 31 patients received saquinavir/atazanavir/ritonavir (Group 2). The primary endpoint for efficacy was the rate of early virological success, defined as plasma viral load <50 copies/mL at week 16. The study is registered with ClinicalTrials.gov (NCT00122603). RESULTS At baseline, median (range) viral load was 4.8 log(10) copies/mL (4.0-5.7) and the median CD4 cell count was 271/mm(3) (197-740). Viral load was <50 copies/mL in 12/30 patients [40%, 95% confidence interval (CI) 23%-58%] and 13/31 patients (42%, 95% CI 25%-59%) at week 16 in Groups 1 and 2, respectively. Patients with failing regimens (viral load >or=400 copies/mL at week 16 or >or=50 copies/mL at week 24) were switched to a standard antiretroviral regimen. At week 48, by an intention-to-treat analysis, 23/30 patients (77%) and 26/31 patients (84%) had plasma HIV-1 RNA <50 copies/mL in Groups 1 and 2, respectively. Four patients discontinued treatment for adverse events, all before week 4. No major changes in the protease gene were detected at treatment failure relative to baseline. Baseline viral load <50 000 copies/mL was the only predictor of virological success at week 16. CONCLUSIONS Ritonavir-boosted dual protease inhibitor regimens targeting only one step of viral replication were insufficient to rapidly suppress plasma HIV RNA to <50 copies/mL in antiretroviral-naive patients with high viral load at baseline.