Background: Residual HIV viremia, defined by low-levels of plasma HIV-RNA with enhanced-sensitivity assays, may persist even in the presence of successful antiretroviral therapy, but little is known about its determinants. Objective: To evaluate rate and determinants of residual viremia in patients who show stable undetectable plasma HIV-1 RNA with conventional assays. Study design: Forty-four multidrug-experienced patients with undetectable levels of HIV-RNA for at least two years under raltegravir-based regimens were evaluated. An ultrasensitive (2.5 copies/ml) real-time PCR method was used to quantify plasma HIV-RNA. Results: After 12 months of salvage treatment, 48.3% of the patients had residual viremia between 2.5 and 37 copies/ml. The proportion of patients with plasma HIV-RNA below 2.5 copies/ml decreased from 51.7% at 12 months to 30.8% at 24 months. Presence of residual viremia was not associated with levels of viremia before starting raltegravir, CD4 counts, HBV or HCV coinfection, or other demographic characteristics, except for the time interval between HIV diagnosis and initiation of antiretroviral therapy: patients with a longer interval (>1 year) were significant less likely to have RNA levels below 2.5 copies/ml at 12 months compared to patients who started within one year from HIV diagnosis (28.6% vs. 73.3%, p=0.027). Conclusion: Half of the patients showing undetectable HIV viremia with conventional assays have low-level viremia with ultrasensitive assays, with no predictive role of viroimmunological status at start of the regimen. The potential influence of the interval between HIV diagnosis and initiation of treatment should be confirmed in subjects with known date of seroconversion.