The association between stress and cancer incidence has been studied for more than seven decades. Despite plausible biological mechanisms and evidence from laboratory studies, findings from clinical research are conflicting. The objective of this study was to examine the association between PTSD and various cancer outcomes. This nation-wide cohort study included all Danish-born residents of Denmark from 1995 to 2011. The exposure was PTSD diagnoses (n = 4131). The main outcomes were cancer diagnoses including: (1) all malignant neoplasms; (2) hematologic malignancies; (3) immune-related cancers; (4) smoking- and alcohol-related cancers; (5) cancers at all other sites. Standardized incidence ratios (SIR) were calculated. Null associations were found between PTSD and nearly all cancer diagnoses examined, both overall [SIR for all cancers = 1.0, 95 % confidence interval (CI) = 0.88, 1.2] and in analyses stratified by gender, age, substance abuse history and time since PTSD diagnosis. This study is the most comprehensive examination to date of PTSD as a predictor of many cancer types. Our data show no evidence of an association between PTSD and cancer in this nationwide cohort.