The schizophrenia diagnosis in Denmark.  A register-based investigation. 
Patients, Danish citizens only, admitted for the first time in 1972 to a Danish psychiatric institution were selected from the national psychiatric register.  To be included, the probands had to have been diagnosed as schizophrenics at least once in the period from their first admission to 1 September 1983.  The study comprised 370 males and 217 females with a total of 5,298 admissions.  The probands' diagnostic pattern during the above period was investigated.  More males (51.9%) than females (39.2%) (P less than 0.01) were diagnosed as schizophrenics during their first admission.  The average period from a patient's first contact with an in-patient institution until schizophrenia was diagnosed for the first time was 2.2 years for females and 1.7 years for males (P less than 0.05).  Personality disorders, reactive psychoses, and not classifiable psychoses were the most frequent diagnoses prior to the first schizophrenia diagnosis.  The diagnostic stability of schizophrenia as main diagnosis, after its first application, was 73.6% for males among a total of 2,539 admissions and 71.2% for females among 1,141 admissions.  There was greater correlation between the latest and former diagnoses than between the first and subsequent diagnoses.  This is valid both when distinguishing between schizophrenia and non-schizophrenia and when focusing on schizophrenia subtypes.  The results are discussed, particularly the problem concerning the selection of representative cohorts for schizophrenia research projects.