Stability of diagnosis in schizophrenia and affective disorders is high. Patients selected according to research criteria for schizophrenia and affective disorders were followed up in a historical prospective study to evaluate their diagnostic stability over a 30- to 40-year period. Our follow-up, lifetime diagnosis, based on blind diagnostic assessment, showed that 92.5% of the personally interviewed schizophrenics were given the follow-up diagnosis of schizophrenia, which was significantly higher than the 78.3% found in affective disorders. However, no significant difference exists when assessment was made on available records of those who died or refused to be interviewed. These stability coefficients are discussed in light of the methodological assumptions involved in a long-term follow-up study. We concluded that the diagnostic stability in schizophrenia and affective disorders were very high and that rigorous diagnostic criteria should be maintained.