BACKGROUND: There are 2 once-daily, fixed-dose-combination, dual-nucleoside analogue tablets: tenofovir 300 mg-emtricitabine 200 mg (TDF-FTC) and abacavir 600 mg-lamivudine 300 mg (ABC-3TC). Which fixed-dose-combination tablet is more effective and safe is uncertain. METHODS: We compared TDF-FTC and ABC-3TC in a randomized, open-label, 96-week trial in which either fixed-dose-combination was substituted for current nucleoside treatments in human leukocyte antigen-B*5701-negative adults with human immunodeficiency virus loads <50 copies/mL. The primary end point was virological failure (consecutive viral load measurements >400 copies/mL, by intention-to-treat). Secondary end points included death, AIDS, adverse events, serious non-AIDS events, metabolic parameters, and body composition. We used exact statistics for differences in proportions, T tests to compare means, and Cox regression for hazard ratios. RESULTS: Of 441 patients who were screened, 357 were treated; 98% were men, the mean age was 45 years, 30% were receiving TDF, 20% were receiving ABC, and 24% were receiving a protease inhibitor. Virological failure was uncommon (5.6% for ABC-3TC and 3.9% for TDF-FTC; difference, 1.7%; 95% confidence interval [CI], -2.8% to 6.1%; [Formula: see text]). No participant developed AIDS, whereas 18 (5%) participants developed a serious non-AIDS event (rate, 2.79 events per 100 person-years; 95% CI, 1.76-4.43), of which 4 were fatal. TDF-FTC was associated with significantly fewer serious non-AIDS events than ABC-3TC (1.2 vs 4.8 events per 100 patient-years; hazard ratio [HR], 0.24; 95% CI, 0.08-0.73; [Formula: see text]), influenced mostly by a lower rate of cardiovascular events (0.3 vs 2.2 events per 100 patient-years; HR, 0.12; 95% CI, 0.02-0.98; [Formula: see text]). TDF-FTC resulted in significantly lower bone mineral density (mean difference in hip t score, 0.16; 95% CI, 0.08-0.23; [Formula: see text]) but not in more fractures. CONCLUSIONS: In this population, TDF-FTC and ABC-3TC had similar virological efficacy, but ABC-3TC was associated with more serious non-AIDS events, particularly cardiovascular events. Clinical trials registration. NCT00192634 .