77 patients with delusional psychoses, regardless of their nosological attribution (except severe organicity), and their first-degree relatives were diagnosed with the Research Diagnostic Criteria (RDC) and the Vienna Research Criteria (VRC). The diagnostic procedure was performed blindly in the relatives. Both criteria were sufficiently capable of identifying a schizophrenic and affective subgroup of patients characterized by the appearance of homotypical secondary cases. Apart from a small RDC schizoaffective group differing in genetic pattern, there exists another large group of nonschizophrenic, nonaffective delusional disorders lacking a genetic link to the above-mentioned diagnoses. In respect to the development of the diagnostic criteria, the results of this study call for the formulation of (1) a narrow definition of schizophrenia (as in the VRC) which is based on thought disorder and affective blunting with the exception of so-called productive symptomatology (delusions, hallucinations); (2) separate criteria for schizoaffective disorders (as in RDC), and (3) a broad and nonrestrictive definition for nonschizophrenic delusional disorders.