The objective was to validate the diagnoses of peripheral arterial disease (PAD) in the legs, obtained from national registers in Denmark.In total, 1435 registered cases of PAD were identified in the Danish National Patient Registry among 57,053 middle aged participants from the Danish Diet, Cancer and Health cohort study. Validation was performed by reviewing all medical records using pre-specified criteria for a diagnosis of PAD.The overall positive predictive value (PPV) of PAD diagnoses was 69.4% [95% confidence interval (CI) 67.0-71.7]. The PPV of diagnoses given in departments of vascular surgery was significantly higher than diagnoses given in other departments: 71.9% (95% CI 69.2-74.4) versus 58.3% (95% CI 52.2-64.2), respectively. In a sub-study, 141 potential cases of PAD also registered in the Danish National Vascular Registry were evaluated, and a PPV of 87.9% (95% CI 81.4-92.4) was found for these diagnoses.More than 30% of the diagnoses of PAD notified in the Danish National Patient Registry were not valid, stressing the importance of validation when using register information for research purposes. In contrast, diagnoses obtained from the Danish National Vascular Registry had a high validity ready for use without further validation.