This study validated hypertension (ICD-8: 401.99) in The National Inpatient Register with reference to the use of the diagnosis in the Occupational Hospitalization Register. A university hospital and a regional hospital were chosen for the evaluation. A sample of case records with the discharge diagnosis essential hypertension and additional case records with other diagnoses were re-coded blindly and independently by two doctors. Cause of admission or admission diagnosis was recorded for essential hypertension cases. The agreement with The National Inpatient Register ranged from 60 to 40%. About half of the cases with the discharge diagnosis "essential hypertension" were admitted to hospital due to hypertension, about a quarter due to diagnoses within "other arteriosclerotic diseases" (ICD-8: 400, 402-440). The misclassification may lead to an underestimation of risks of hypertension in various occupations in the Occupational Hospitalization Register or bias the occupational risk pattern of essential hypertension to become more alike that of arteriosclerotic diseases.