Background: Integrase strand transfer inhibitors (INSTIs) are recommended components of initial antiretroviral therapy with two nucleoside reverse transcriptase inhibitors. Bictegravir is a novel, potent INSTI with a high in-vitro barrier to resistance and low potential as a perpetrator or victim of clinically relevant drug-drug interactions. We aimed to assess the efficacy and safety of bictegravir coformulated with emtricitabine and tenofovir alafenamide as a fixed-dose combination versus coformulated dolutegravir, abacavir, and lamivudine. Methods: We did this double-blind, multicentre, active-controlled, randomised controlled non-inferiority trial at 122 outpatient centres in nine countries in Europe, Latin America, and North America. We enrolled HIV-1 infected adults (aged ≥18 years) who were previously untreated (HIV-1 RNA ≥500 copies per mL); HLA-B*5701-negative; had no hepatitis B virus infection; screening genotypes showing sensitivity to emtricitabine, tenofovir, lamivudine, and abacavir; and an estimated glomerular filtration rate of 50 mL/min or more. Participants were randomly assigned (1:1), via a computer-generated allocation sequence (block size of four), to receive coformulated bictegravir 50 mg, emtricitabine 200 mg, and tenofovir alafenamide 25 mg or coformulated dolutegravir 50 mg, abacavir 600 mg, and lamivudine 300 mg, with matching placebo, once daily for 144 weeks. Randomisation was stratified by HIV-1 RNA (≤100 000 copies per mL, > 100 000 to ≤400 000 copies per mL, or > 400 000 copies per mL), CD4 count ( <50 cells per μL, 50-199 cells per μL, or ≥200 cells per μL), and region (USA or ex-USA). Investigators, participants, and study staff giving treatment, assessing outcomes, and collecting data were masked to group assignment. The primary endpoint was the proportion of participants with plasma HIV-1 RNA less than 50 copies per mL at week 48, as defined by the US Food and Drug Administration snapshot algorithm, with a prespecified non-inferiority margin of -12%. All participants who received one dose of study drug were included in primary efficacy and safety analyses. This trial is registered with ClinicalTrials.gov, number NCT02607930. Findings: Between Nov 13, 2015, and July 14, 2016, we randomly assigned 631 participants to receive coformulated bictegravir, emtricitabine, and tenofovir alafenamide (n=316) or coformulated dolutegravir, abacavir, and lamivudine (n=315), of whom 314 and 315 patients, respectively, received at least one dose of study drug. At week 48, HIV-1 RNA less than 50 copies per mL was achieved in 92·4% of patients (n=290 of 314) in the bictegravir, emtricitabine, and tenofovir alafenamide group and 93·0% of patients (n=293 of 315) in the dolutegravir, abacavir, and lamivudine group (difference -0·6%, 95·002% CI -4·8 to 3·6; p=0·78), demonstrating non-inferiority of bictegravir, emtricitabine, and tenofovir alafenamide to dolutegravir, abacavir, and lamivudine. No individual developed treatment-emergent resistance to any study drug. Incidence and severity of adverse events was mostly similar between groups except for nausea, which occurred less frequently in patients given bictegravir, emtricitabine, and tenofovir alafenamide than in those given dolutegravir, abacavir, and lamivudine (10% [n=32] vs 23% [n=72] ; p <0·0001). Adverse events related to study drug were less common with bictegravir, emtricitabine, and tenofovir alafenamide than with dolutegravir, abacavir, and lamivudine (26% [n=82] vs 40% [n=127] ), the difference being driven by a higher incidence of drug-related nausea in the dolutegravir, abacavir, and lamivudine group (5% [n=17] vs 17% [n=55] ; p <0·0001). Interpretation: At 48 weeks, coformulated bictegravir, emtricitabine, and tenofovir alafenamide achieved virological suppression in 92% of previously untreated adults and was non-inferior to coformulated dolutegravir, abacavir, and lamivudine, with no treatment-emergent resistance. Bictegravir, emtricitabine, and tenofovir alafenamide was safe and well tolerated with better gastrointestinal tolerability than dolutegravir, abacavir, and lamivudine. Because coformulated bictegravir, emtricitabine, and tenofovir alafenamide does not require HLA B*5701 testing and provides guideline-recommended treatment for individuals co-infected with HIV and hepatitis B, this regimen might lend itself to rapid or same-day initiation of therapy in the clinical setting. Funding: Gilead Sciences. © 2017 Elsevier Ltd.