Background: Combination ART (cART)-related toxicities and costs have prompted the need for treatment simplification. The ATLAS-M trial explored 48 week non-inferior efficacy of simplification to atazanavir/ritonavir + lamivudine versus maintaining three-drug atazanavir/ritonavir-based cART in virologically suppressed patients. Methods: We performed an open-label, multicentre, randomized, non-inferiority study, enrolling HIV-infected adults on atazanavir/ritonavir+two NRTIs, with stable HIV-RNA 200 cells/mm3. Main exclusion criteria were hepatitis B virus coinfection, past virological failure on or resistance to study drugs, recent AIDS and pregnancy. Patients were randomly assigned 1:1 to either switch to 300mg of atazanavir/ 100mg of ritonavir once daily and 300mg of lamivudine once daily (atazanavir/ritonavir+lamivudine arm) or to continue the previous regimen (atazanavir/ritonavir+two NRTIs arm). The primary study outcome was the maintenance of HIV-RNA