Objectives: To assess whether low-dose ritonavir-boosted darunavir (darunavir/r) in combination with two NRTIs could maintain virological suppression in patients on a standard regimen of darunavir/r + two NRTIs. Design: A multicentre, Phase II, non-comparative, single-arm, open-label study. Setting: Tertiary care hospitals in France. Subjects: One hundred HIV-1-infected adults with no darunavir or NRTI resistance-associated mutations (RAMs) and a plasma HIV RNA level ≤50 copies/mL for ≥12 months on once-daily darunavir/r (800/100 mg) + two NRTIs for ≥6 months were switched to darunavir/r 400/100 mg with the same NRTIs. Primary outcome measure: Proportion of patients with treatment success: plasma HIV RNA level ≤50  copies/mL up to 48 weeks without any change in the study regimen, in a modified ITT (mITT) analysis. Results: At baseline, most patients were male (78%), with a median age of 43 years, median duration of HIV RNA ≤50 copies/mL of 35 months and median CD4 T cell count of 633 cells/mm3. Seventy-six percent received tenofovir/emtricitabine and 24% abacavir/lamivudine. Five patients were excluded from the mITT analysis. The rate of treatment success through to week 48 was 91.6% (87/95; 95% CI 84.1%-96.3%). No RAM was detected in three amplifiable genotypes. A total of 212 adverse events (AEs) occurred in 64 patients (64%); 9 AEs were serious, none leading to treatment discontinuation. Conclusions: In HIV-infected patients well suppressed with darunavir/r (800/100 mg) and two NRTIs, a reduction of the darunavir dose to 400 mg/day maintained virological efficacy and was safe over 48 weeks.