Studies validating the clinical diagnoses of affective disorder recorded in case registers against research diagnostic criteria do not exist. In the present study, a random sample of 100 patients was selected among 21,734 patients who were recorded in the Danish Psychiatric Central Register with a diagnosis of manic-depressive psychosis at their first admission to psychiatric ward in a period from January 1, 1971 to December 31, 1993. Case notes from alt over Denmark were reviewed for all 100 patients and diagnoses were made with the use of OPCRIT. Patients who were still alive were contacted and interviewed face to face or by telephone. In total, 95 out of the 100 patients received an ICD-10 diagnosis of affective disorder computed with OPCRIT and confirmed at the interviews. Other clinical characteristics such as the age at onset and the number of affective episodes estimated from the register corresponded well with information from the case notes and the interviews.