Background: We pooled the results of three randomized trials that compared the efficacy of PI/r monotherapy and standard triple therapy as maintenance therapy and evaluated: 1) the distribution of ultrasensitive viral load (USVL) at week 96 (W96), 2) factors associated with virological success (VL<50 copy/mL) at W96, and 3) factors associated with USVL<1 copy/ml (c/ml) at W96. Methods: Virological failure was defined as two consecutive measurements of HIV-1 RNA viral load >50 copies/mL and was analyzed in intention-to-treat. USVL was measured with commercial standard Roche assay. A logistic model was used to investigate which variables were predictive of VF. The Fisher exact test was used to investigate differences in USVL at W96. Results: Among 609 patients, 73% were male with a median age of 44.4 years (IQR 39.8-52.1), the treatment duration was four years, (2.4-7.6), baseline CD4/CD8 ratio 0.8 (0.6-1.10), baseline CD4 cell count 564/mm3 (422-707), and 59% presented a baseline USVL<1 copy/mL. At W96, the proportion of USVL<1 copy/mL was significantly lower for PI/r monotherapy than triple therapy (65% versus 74%; p=0.04). Overall, baseline USVL<1 copy/mL, triple therapy, and being female were associated with an USVL<1 copy/mL at W96 (p<0.0001, p=0.049 and p=0.006). For PI/r monotherapy, receiving DRV/r rather than LPV/r was associated with an USVL<1 copy/mL at W96 (p=0.03). Factors associated with virological success at W96 were higher baseline CD4 cell count (p=0.034) and baseline USVL<1 copy/mL (p=0.0005). Conclusion: Although PI/r monotherapy is not widely recommended, this strategy is still sometimes used and USVL determination for virologically-controlled patients may help to select the best candidates for PI/r monotherapy.Read less <