OBJECTIVE To analyze the safety and effectiveness of abacavir, lamivudine, and zidovudine (ABC/3TC/ZDV) in antiretroviral therapy (ART)-naive HIV-infected patients. DESIGN Retrospective observational cohort study. METHODS We analyzed all consecutive ART-naive HIV-infected patients who initiated ABC/3TC/ZDV in 71 centers throughout Spain and had a clinical visit and laboratory data at least 16 weeks after initiating this regimen. We assessed safety, mortality, new AIDS-defining conditions (ADCs) and treatment failure, the latter defined by any of the following: (1) reduction in plasma HIV-1 viral load (pVL) <1 log during the first 12 weeks of ART, unless it was less than the lower limit of quantification (LOQ); (2) failure to achieve a pVL <LOQ after 24 weeks of ART; and (3) rebound to 2 consecutive pVLs > or = LOQ after achieving a pVL <LOQ. RESULTS A total of 730 patients were included, median patient age was 37 years, prior ADCs occurred in 20%, median pVL was 4.76 log, and median CD4 count was 255 cells/mm; 109 (14.9%) patients had <100 CD4 cells/mm. After a median follow-up of 50.5 weeks (interquartile ratio: 28-78), 104 (14.25%) patients discontinued therapy because of adverse events and 36 (4.93%) had a suspected hypersensitivity reaction to ABC. The frequency of treatment failure according to an intention-to-treat (ITT) analysis of observed data was 14.4%. In a more rigorous approach considering losses to follow-up and interruptions or switches of therapy as failures, however, the frequency of treatment failure was 22.92%. Factors independently associated with treatment failure by observed data ITT analysis were adherence <90% (hazard ratio [HR] = 4.248, 95% confidence interval [CI]: 2.640 to 6.833), methadone use (HR = 2.116, 95% CI: 1.180 to 3.797), baseline pVL (HR = 1.651, 95% CI: 1.190 to 2.292 per log), and prior ADC (HR = 1.639, 95% CI: 1.009 to 2.662). CONCLUSION The triple-nucleoside regimen of ABC/3TC/ZDV is a reasonable option for ART-naive patients with a pVL <100,000 copies/mL in whom, for any reason, preferred regimens are not advisable, even in patients with a baseline CD4 cell count <100 cells/mm.