Background: Although antiretroviral therapy (ART) is known to be protective against HIV-related mortality, the expected magnitude of effect is unclear because existing estimates of the effect of ART may not directly generalize to recently HIV-diagnosed persons. Methods: In this study, we estimated 5-year mortality risks for immediate versus no ART initiation among patients (n=12 547) in the Centers for AIDS Research Network of Integrated Clinical Systems (CNICS) using the complement of adjusted Kaplan-Meier survival functions. We subsequently standardized estimates to persons diagnosed with HIV in the USA between 2009 and 2011, who were enumerated using national surveillance data. Results: The 5-year mortality, had all patients in the CNICS immediately initiated ART, was 10.6% [95% confidence interval (CI): 9.3%, 11.9%] compared with 28.3% (95% CI: 19.1%, 37.5%) had ART initiation been delayed at least 5 years. The 5-year mortality risk difference due to ART among patients in the CNICS was -17.7% (95% CI: -27.0%, -8.4%). Based on methods for generalizing an estimate from a study sample to a different target population, the expected risk difference due to ART initiation among recently HIV-diagnosed persons in the USA was -19.1% (95% CI: -30.5%, -7.8%). Conclusions: Immediate ART initiation substantially lowers mortality among persons in the CNICS and this benefit is expected to be similar among persons recently diagnosed with HIV in the USA. We demonstrate a method by which concerns about generalizability can be addressed and evaluated quantitatively.