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Background: Most information on the incidence and prognosis of dementia comes from small studies with limited precision and generalisability. Nationwide registers can be an alternative source of information, but only when the diagnosis is validly recorded. We assessed the validity of the Dutch Hospital Discharge Register (HDR)., Methods: HDR data on dementia diagnoses (ICD-9 codes 290.0; 290.1; 290.3; 290.4; 294.1; 331.0; 331.1; 331.82) in a university medical centre in the Netherlands were collected. Diagnoses were verified by using hospital medical records. Positive predictive values (PPVs) were calculated. Multivariate logistic regression models were used to evaluate determinants of inaccuracy in discharge diagnoses., Results: A sample of the HDR data was used for this study (n = 340). PPV was 93.2% for overall dementia, indicating confirmation of 93.2% of HDR dementia diagnoses by the medical records. The accuracy of the diagnosis of overall dementia in patients aged ≥ 65 years was significantly higher compared with younger patients (PPV 95.5 % vs. 67.9%; p = 0.0001). There was no difference in the accuracy of the diagnosis between men and women and accuracy was not influenced by type of admission, comorbidity and polypharmacy., Conclusion: The results of this study show a high validity of the diagnosis of overall dementia in the HDR, making this register of great value for further nationwide research on dementia.