--- - |2+ Background Prevention of PTSD requires identification of subpopulations contributing most to the population burden of PTSD. This study examines the relative contribution of subthreshold PTSD and probable PTSD on future PTSD in a representative military cohort. Methods We analyze data on 3,457 U.S. National Guard members from the state of Ohio, assessed by telephone annually from 2008 to 2014. At each wave, participants were classified into one of three groups based on the PTSD Checklist: probable PTSD (DSM‐IV‐TR criteria), subthreshold PTSD (Criterion A1, at least one symptom in each cluster, symptom lasting longer than 30 days, and functional impairment), and no PTSD. We calculated the exposure rate, risk ratio (RR), and population attributable fraction (PAF) to determine the burden of future probable PTSD attributable to subthreshold PTSD compared to probable PTSD. Results The annualized prevalence of subthreshold PTSD and probable PTSD was respectively 11.9 and 5.0%. The RR for probable PTSD was twice as great among respondents with probable PTSD the prior interview than that of those with subthreshold PTSD (7.0 vs. 3.4); however, the PAF was considerably greater in participants with subthreshold PTSD the prior interview (PAF = 35%; 95% confidence interval (CI) = 26.0–42.9%) than in those with probable PTSD (PAF = 28.0%; 95% CI = 21.8–33.8%). Results were robust to changes in subthreshold PTSD definition. Conclusions Subthreshold PTSD accounted for a substantial proportion of this population's future PTSD burden. Population‐based preventive interventions, compared to an approach focused exclusively on cases of diagnosable PTSD, is likely to affect the greatest reduction in this population's future PTSD burden. - Depression and Anxiety, EarlyView.