RATIONALE: Studies have suggested heightened anxiety among adults with asthma; the mechanism of this association is not known. OBJECTIVES: To determine the association between post-traumatic stress disorder (PTSD) symptoms and asthma among adults, and to examine if this association is due to confounding by environmental and genetic factors. METHODS: Data were obtained from twins in the Vietnam Era Twin Registry, which includes male veteran twin pairs born between 1939 and 1956 who served during the Vietnam era (1965-1975). Measurements included a symptom scale for PTSD, history of a doctor diagnosis of asthma, and sociodemographic and health confounding factors. Co-twin control analytic methods used mixed-effects logistic regression to account for the paired structure of the twin data and to examine the association between PTSD symptoms and asthma in all twins. Separate analyses were conducted within twin pairs and according to zygosity. MEASUREMENTS AND MAIN RESULTS: PTSD symptoms were associated with a significantly increased likelihood of asthma (P(trend) < 0.001) even after adjustment for confounding factors. Among all twins, those in the highest quartile of PTSD symptoms were 2.3 times as likely (95% confidence interval, 1.4-3.7) to have asthma compared with those in the lowest quartile. These findings persist when examined within twin pairs and when stratified by zygosity. CONCLUSIONS: Symptoms of PTSD were associated with an elevated prevalence of asthma. Even after careful adjustment for familial/genetic factors and other potential confounding factors, an association between PTSD symptoms and asthma remains. Efforts to understand this comorbidity may be useful in identifying modifiable environmental risk factors contributing to this pattern and therefore in developing more effective prevention and intervention strategies.