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Summary Introduction We examined the positive predictive value of diagnoses of acute exacerbation of chronic obstructive pulmonary disease (COPD) in the Danish National Patient Registry. We also examined the negative predictive value of acute pneumonia or respiratory failure discharge diagnoses for absence of underlying COPD. Methods We identified all patients aged 30 years or older with acute hospital admission in Denmark from January 1st to December 31st 2008. Physicians at 34 Danish hospitals retrieved and reviewed medical records for 1581 patients with a discharge diagnosis of COPD, and for 1546 patients with a discharge diagnosis of either pneumonia or respiratory failure but no COPD diagnosis. Presence of COPD was assessed based on medical history, clinical symptoms and findings, and spirometry results. Results The overall positive predictive value for COPD was 92% (95% confidence interval [CI] = 91–93%). Among patients coded with pneumonia or respiratory failure but not COPD, 19% (95% CI = 17–21%) had COPD, corresponding to a negative predictive value for COPD of 81% (95% CI = 79–83%). Conclusions The positive predictive value of acute COPD discharge diagnoses in the Danish National Patient Registry is high. At the same time, there is a substantial underrecording of COPD during hospitalizations with other acute respiratory disorders like pneumonia and respiratory failure.