Switches from lopinavir/ritonavir (LPV/r) to either atazanavir/ritonavir (ATV/r) or unboosted ATV (ATV) are increasingly common in clinical practice, but data on outcome comparison between these two simplification strategies are very limited. Methods. Multicenter, observational, retrospective study. Data were collected from five Italian clinics. The objective of the study was to investigate the utcome of LPV/r simplification with ATV/r or ATV and to identify factors predicting virological rebound. Patients who switched from LPV/r to ATV/r or ATV with an HIV-RNA value30000 copies/mL (28% vs 6%, p=0.014). Replacing lopinavir/r with ATV or ATV/r yielded similar rates of virological rebound. Viral load at the initiation of lopinavir/r may be useful in driving the choice between ATV/r and ATV
