OBJECTIVE ODIN (Once-daily Darunavir In treatment-experieNced patients) was a phase III, 48-week, open-label study comparing once-daily vs. twice-daily darunavir/ritonavir (DRV/r) in treatment-experienced patients with no DRV resistance-associated mutations (RAMs) at screening. METHODS Patients with no DRV RAMs and receiving stable HAART for at least 12 weeks were stratified by HIV-1 RNA (≤ or > 50 000 copies/ml) and randomized to once-daily DRV/r 800/100 mg or twice-daily DRV/r 600/100 mg and an optimized background regimen (≥2 nucleoside reverse transcriptase inhibitors). Primary objective was to demonstrate noninferiority of once-daily vs. twice-daily DRV/r in confirmed virologic response (HIV-1 RNA < 50 copies/ml) at week 48. RESULTS Five hundred and ninety patients received once-daily (n = 294) or twice-daily (n = 296) DRV/r. Mean baseline HIV-1 RNA was 4.16 log10 copies/ml; median CD4 cell count was 228 cells/μl; and 53.9% had previously used at least one protease inhibitor. At week 48, 72.1% of once-daily and 70.9% of twice-daily patients achieved HIV-1 RNA less than 50 copies/ml (intent-to-treat/time-to-loss of virologic response). The difference in response between once-daily and twice-daily arms was 1.2% (95% confidence interval -6.1 to 8.5%; P < 0.001), establishing noninferiority of once-daily DRV/r versus twice-daily DRV/r. Median CD4 cell count increase was 100 (once-daily) and 94 cells/μl (twice-daily). Virologic failure rate was low and similar for both arms; only one patient (once-daily arm) developed primary protease inhibitor mutations. Once-daily DRV/r had a lower incidence of grade 2-4 triglyceride increases (5.2 vs. 11.0%, P < 0.05). CONCLUSIONS Once-daily DRV/r 800/100 mg was noninferior in virologic response to twice-daily DRV/r 600/100 mg at 48 weeks in treatment-experienced patients with no DRV RAMs, and with a more favorable lipid profile. These findings support use of once-daily DRV/r in this population.