To assess the efficacy and the tolerability of once-daily (QD) versus twice-daily (BID) nevirapine (NVP)-based highly active antiretroviral therapy (HAART) in virologically suppressed, HIV-positive patients switched from protease inhibitor (PI)-based HAART. Eligible patients were enrolled in the multicenter trial if HIV RNA levels were <50 copies/mL for ³6 months prior. Patients were switched from a PI to NVP 200 mg BID for 2 months, and then randomized to continue with that regimen (group A) or NVP 400 mg QD (group B) for a further 10 months. Virological efficacy (primary endpoint) and tolerability/toxicity were evaluated according to an intention-to-treat analysis. A total of 126 patients (63 per group) were enrolled. Withdrawals from the study (any reason) numbered 15 in group A and 14 in B, virological failures numbered 5 and 2, respectively, and there were 4 cases of adverse events in each group (all p = NS). Mean alanine aminotransaminase (ALT) and gamma-glutamyl transpeptidase (γ-GT) level increases were significant for the whole cohort (33.2±22.9 to 43.3±29.1, p < 0.001; 57.3±72 to 109±131 U/L, p < 0.0002, respectively), but there were no differences between the two groups. Apparently, no significant differences between the QD and BID NVP groups were found, in terms of virological failures or tolerability/toxicity. The switch to NVP may be safely pursued with a QD schedule.