Background: The Veterans Aging Cohort Study (VACS) index is a weighted combination of age and 8 clinical variables. It has been well correlated with all-cause mortality among HIV-infected patients. The US Military HIV Natural History Study (NHS) cohort provides a different validation population profile, being younger and healthier. A significant portion of the US HIV population is similarly composed; so, evaluation of the VACS index in this population is of great interest. Methods: NHS subjects have medical history and laboratory data collected at 6-month visits. We performed an external validation of the VACS index in the NHS evaluating correlation, discrimination, and calibration for all-cause mortality after highly active antiretroviral therapy initiation (HI). We then tested whether combining longitudinal VACS index values at different time points improves prediction of mortality. Results: The VACS index at 1 year after HI was well correlated with all-cause mortality (Harrell c statistic 0.78), provided good discrimination (log-rank P < 0.05), and was marginally well calibrated using Brier score. Accounting for VACS index at HI and 6 months after HI significantly improved a standard model, including only the VACS index at 1 year after HI (net reclassification improvement = 25.2%, 95% CI: 10.9% to 48.9%). Conclusions: The VACS index was well correlated and provided good discrimination with respect to all-cause mortality among highly active antiretroviral therapy initiating subjects in the NHS. Moderate overprediction of mortality in this young, healthy population suggests minor recalibration that could improve fit among similar patients. Considering VACS index at HI and 6 months improved outcome prediction and allowed earlier risk assessment.