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STUDY DESIGN Longitudinal co-twin control study of the Vietnam-Era Twin Registry. OBJECTIVE To examine the association of post-traumatic stress disorder (PTSD) symptoms with incident chronic back pain (CBP), while controlling for genetic factors and early family environment. SUMMARY OF BACKGROUND DATA It is unknown whether PTSD symptoms are associated with an increased incidence of CBP. METHODS In 2010-2012 a baseline survey was undertaken as part of a large-scale study of PTSD. Study participants completed the PTSD Symptom Checklist (PCL) and a self-report measure of CBP. In 2015-2017 a follow-up survey was sent to all 171 monozygotic (MZ) twin pairs (342 individuals) where both co-twins had no history of CBP at baseline, but only one co-twin in the pair met criteria for having current PTSD symptoms (one twin with PCL <30 and the co-twin with PCL≥30). No other inclusion/exclusion criteria were applied. CBP at 5-year follow-up was defined as back pain of duration ≥3 months in the low back or mid/upper back. Covariates included age, race, education, income, Veterans Affairs health care use, disability compensation, smoking, body mass index, and depression. Statistical analysis estimated the cumulative incidence of CBP according to baseline PTSD symptoms. Risk ratios (RRs) and 95% confidence intervals (CI) were estimated in matched-pair co-twin control analyses adjusting for familial factors. RESULTS Among 227 males completing 5-year follow-up, including 91 MZ twin pairs, the mean age was 62 years. Five-year incidence of CBP in those without and with baseline PTSD symptoms was 40% and 60%, respectively. Baseline PTSD symptoms were significantly associated with incident CBP in crude and multivariable-adjusted within-pair analyses (RR 1.6 [95% CI 1.2-2.1]; p = 0.002). CONCLUSIONS PTSD symptoms were associated with an increased incidence of CBP, without confounding by genetic factors or early family environment. PTSD symptoms may be a modifiable risk factor for prevention of CBP. LEVEL OF EVIDENCE 3.