Maraviroc (UK-427,857), a selective and reversible CCR5 coreceptor antagonist, has been shown to be active in vitro against a wide range of clinical isolates (including those resistant to existing classes). In HIV-1 infected patients, maraviroc (UK-427,857) given as monotherapy for 10 days reduced HIV-1 viral load by up to 1.6 log, consistent with currently available agents. Safety and toleration have been studied in over 400 subjects for up to 28 days at 300 mg twice daily. No significant effects were seen on the QTc interval. The goal of this study is to compare the safety and efficacy of maraviroc (UK-427,857) versus efavirenz, when each are combined with two other antiretroviral agents, in patients who are previously naive to antiretroviral therapy. This study will involve approximately 200 centers from around the world to achieve a total randomized subject population of 1071 subjects. Patients will be randomly assigned to one of three groups: maraviroc (UK-427,857) 300 mg once daily added to zidovudine/lamivudine (300 mg/150 mg twice daily), Maraviroc (UK-427,857) 300 mg twice daily added to zidovudine/lamivudine (300 mg/150 mg twice daily) or efavirenz (600 mg once daily) added to zidovudine/lamivudine (300 mg/150 mg twice daily). The study will enroll over approximately an 18 month period (5 months Phase 2b run-in, 13 months Phase 3) with 96 weeks of treatment. This may be extended for an additional 3 years depending on the results at 96 weeks. Physical examinations will be performed at study entry, weeks 4, 8, 12, 16, 20, 24, 32, 40, 48, 60, 72, 84 and 96. Blood samples will also be taken at study entry, weeks 2, 4, 8, 12, 16, 20, 24, 32, 40, 48, 60, 72, 84 and 96. Additionally, blood samples will be drawn twice, at least 30 minutes apart, at weeks 2 and 48 for maraviroc (UK-427,857) pharmacokinetic analysis. As part of this clinical study a blood sample will be taken for non-anonymized pharmacogenetic analysis. Patients will undergo a 12-lead electrocardiogram at study entry, weeks 24, 48 and 96. A computerized tomography (CT) scan will also be performed, at selected centers, at study entry and week 96. Patients will be asked to complete a symptom distress questionnaire at study entry, weeks 12, 24, 48 and 96.