IMPORTANCE: Posttraumatic stress disorder (PTSD) is a common, debilitating mental disorder that has been associated with type 2 diabetes mellitus (T2D) and its risk factors, including obesity, in cross-sectional studies. If PTSD increases risk of incident T2D, enhanced surveillance in high-risk populations may be warranted. OBJECTIVE: To conduct one of the first longitudinal studies of PTSD and incidence of T2D in a civilian sample of women. DESIGN, SETTING, AND PARTICIPANTS: The Nurses' Health Study II, a US longitudinal cohort of women (N = 49 739).We examined the association between PTSD symptoms and T2D incidence over a 22-year follow-up period. MAIN OUTCOMES AND MEASURES: Type 2 diabetes, self-reported and confirmed with self-report of diagnostic test results, symptoms, and medications, a method previously validated by physician medical record review. Posttraumatic stress disorder was assessed by the Short Screening Scale for DSM-IV PTSD.We examined longitudinal assessments of body mass index, smoking, alcohol intake, diet quality, physical activity, and antidepressant use as mediators of possible increased risk of T2D for women with PTSD. The study hypothesis was formulated prior to PTSD ascertainment. RESULTS: Symptoms of PTSD were associated in a dose-response fashion with T2D incidence (1-3 symptoms: hazard ratio, 1.4 [95%CI, 1.2-1.6]; 4 or 5 symptoms; hazard ratio, 1.5 [95%CI, 1.3-1.7]; 6 or 7 symptoms: hazard ratio, 1.8 [95%CI, 1.5-2.1]). Antidepressant use and a higher body mass index associated with PTSD accounted for nearly half of the increased risk of T2D for women with PTSD. Smoking, diet quality, alcohol intake, and physical activity did not further account for increased risk of T2D for women with PTSD. CONCLUSIONS AND RELEVANCE: Women with the highest number of PTSD symptoms had a nearly 2-fold increased risk of T2D over follow-up than women with no trauma exposure. Health professionals treating women with PTSD should be aware that these patients are at risk of increased body mass index and T2D. Comprehensive PTSD treatment should be expanded to address the health behaviors that contribute to obesity and chronic disease in affected populations.