Background: The generalizability of antiretroviral therapy (ART) clinical trial efficacy findings to routine care settings is not well studied. We compared the relative effectiveness of initial ART regimens estimated in AIDS Clinical Trial Group (ACTG) randomized controlled trials with that among patients receiving ART at Antiretroviral Therapy Cohort Collaboration (ART-CC) study sites. Methods: Treatment-naive HIV-infected patients initiating identical ART regimens in ACTG trials (A5095 and A5142) and at 15 ART-CC cohort study sites were included. Virological failure (HIV-1 RNA >200 copies/mL) at 24 and 48 weeks, incident AIDS-defining events and mortality were measured according to study design (ART-CC cohort vs. ACTG trial) and stratified by third drug [abacavir (ABC), efavirenz (EFV), and lopinavir/r (LPV/r)]. We used logistic regression to estimate and compare odds ratios (OR) for virological failure between different regimens and study designs, and used Cox models to estimate and compare hazard ratios for AIDS and death. Results: Compared with patients receiving ABC, those receiving EFV had roughly half the odds of 24-week virologic failure (>200 copies/mL) in both ACTG 5095 (OR = 0.53, 95% confidence interval: 0.36 to 0.79) and ART-CC (0.46, 0.37 to 0.57). Virologic superiority of EFV (vs. ABC) seemed comparable in ART-CC and ACTG 5095 (ratio of ORs 0.86, 95% confidence interval: 0.54 to 1.35). Odds ratios for 48-week virologic failure, comparing EFV with LPV/r, were also comparable in ACTG 5142 and ART-CC (ratio of ORs: 0.87, 0.45 to 1.69). Conclusions: Between ART regimen virologic efficacy of third drugs ABC, EFV, and LPV/r observed in the ACTG 5095 and 5142 trials seem generalizable to the routine care setting of ART-CC clinical cohorts. © 2011 Lippincott Williams & Wilkins.