Objectives: Posttraumatic Stress Disorder (PTSD) is highly comorbid with chronic pain conditions that often co-occur such as migraine headaches, temporomandibular disorder, irritable bowel syndrome, fibromyalgia, chronic fatigue syndrome, chronic prostatitis/chronic pelvic pain syndrome, and tension headaches. Using a genetically-informative sample, the current study evaluated the genetic and environmental factors contributing to the co-occurrence of PTSD and chronic pain conditions. Methods: Data from 4,680 male twins in the Vietnam Era Twin Registry were examined. Biometric modeling was used to estimate genetic and environmental variance components and genetic and environmental correlations between PTSD and multiple chronic pain conditions. Results: Heritability was estimated at 43% (95% CI: 15–63%) for PTSD, 34% (95% CI: 27–41%) for the combined history of any one or more pain condition. Specific pain condition heritabilities ranged from 15% (95% CI: 0 – 48%) for tension headaches to 41% (95% CI: 27 – 54%) for migraine headaches. Environmental influences accounted for the remaining variance in pain conditions. The genetic correlation between PTSD and combined history of any one or more pain condition was rg = 0.61 (95% CI: 0.46; 0.89) and ranged for individual pain conditions from rg = 0.44 (95% CI: 0.24; 0.77) for migraine headache to rg = 0.75 (95% CI: 0.52; 1.00) for tension headaches. Conclusions: PTSD and chronic pain conditions are highly comorbid, and this relationship can be explained by both genetic and environmental overlap. The precise mechanisms underlying these relationships are likely diverse and multifactorial.