Objective: Maraviroc, a chemokine co-receptor type 5 (CCR5) antagonist, has demonstrated comparable efficacy and safety to efavirenz, each in combination with zidovudine/lamivudine, over 96 weeks in the Maraviroc vs. Efavirenz Regimens as Initial Therapy (MERIT) study. Here we report 5-year findings. Design: A randomized, double-blind, multicenter phase IIb/III study with an open-label extension phase. Methods: Treatment-naive patients with CCR5-tropic HIV-1 infection (Trofile) received maraviroc 300mg twice daily or efavirenz 600mg once daily, and zidovudine/lamivudine 300 mg/150mg twice daily. After the last patient's week 96 visit, the study was unblinded and patients could enter a nominal 3-year open-label phase. Endpoints at the 5-year nominal visit (week 240) included proportion of patients (CCR5 tropism reconfirmed by enhanced sensitivity Trofile) with viral load (plasma HIV-1 RNA) below 50 and 400 copies/ml, and change from baseline in CD4+ cell count, as well as safety. Results: The proportion of patients maintaining viral load below 50 copies/ml was similar between treatment arms throughout the study and at week 240 (maraviroc 50.8% vs. efavirenz 45.9%). Maraviroc-treated patients had a greater increase from baseline in mean CD4+ cell count than efavirenz-treated patients at week 240 (293 vs. 271 cells/μl, respectively). Fewer patients on maraviroc vs. efavirenz experienced treatment-related adverse events (68.9 vs. 81.7%) and discontinued as a result of any adverse event (10.6 vs. 21.3%). Conclusion: Maraviroc maintained similar long-term antiviral efficacy to efavirenz over 5 years in treatment-naive patients with CCR5-tropic HIV-1. Maraviroc was generally well tolerated with no unexpected safety findings or evidence of long-term safety concerns. © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins.