As a result of information about coding practice in a regional hospital, the Norwegian Board of Health carried out a special supervision of ear, nose and throat (ENT) departments and neurological departments during the autumn of 2003 and the spring of 2004. The aim was to investigate whether there was accordance between information in the patient records and the chosen diagnoses and codes.Three hospitals were examined. Information in 100 selected patient records from each department was compared with diagnoses and codes. The codes reported to the Norwegian Patient Register (NPR) from two hospitals were compared with codes in the patient records.Incorrect diagnoses had been systematically recorded in one ENT department. Incorrect codes had been recorded in all the departments, especially the neurological departments. Many inconsistencies between codes in the patient records and those reported to NPR were found.Wrong diagnoses were seldom made, but many diagnoses were unprecise because only ICD-10 diagnoses were used. The rules of coding hospital stays were frequently not used. Many of the codes reported to NPR had been altered, without medical basis and without being noted in the patients' records.