Objective To describe the immunological and virological outcome, and the factors associated to discontinuation in patients switching to a regimen containing efavirenz (EFV), nevirapine (NVP) or abacavir (ABC) after long-term viral suppression under protease inhibitor-including HAART. Design Observational study at three outpatient clinics for HIV care in Italy. Methods Patients with HIV RNA <80 copies/ml and CD4 >200 cells/ml for at least 6 months on a protease inhibitor-containing treatment who switched to NVP, EFV or ABC were included in the study. End-points were immunological failure, virological failure and discontinuation due to toxicity. Survival analyses were performed to find out any independent variables predictive of reaching the end-points. Results 177 patients were enrolled; 85 started EFV, 54 NVP and 38 ABC as part of the simplification regimen. 16/159 patients experienced immunological failure: the variables associated to CD4 count decrease were HIV RNA set point value (HR 2.32 for each log₁₀ copies more, P=0.040) and intolerance/toxicity as reason for simplification (HR 3.96, P=0.05). 13/151 subjects showed virological failure; an AIDS diagnosis (HR 6.04, P=0.021) and the use of NVP (HR 7.98, P=0.027) were associated to a worse virological outcome, while patients naive before HAART showed a lower risk of failure (HR 0.008, P=0.007). 16/177 patients discontinued simplification regimen due to toxicity; longer HAART duration before switch was associated to risk reduction (HR 0.92, P=0.004). Conclusions Simplification is safe and effective, but it should be offered to patients with shorter treatment duration, and in good clinical and immunovirological conditions.