A total 215 first-degree relatives of 88 twin probands with schizophrenia, mood disorders and nonaffective psychoses were studied. The twins' parents and siblings were personally interviewed with structured diagnostic instruments and diagnosed in accordance with DSM-III-R criteria. The first-degree relatives were interviewed by interviewers who were blind to the twins' diagnoses. Schizophrenia and schizotypal personality disorder were significantly more frequent in first-degree relatives of schizophrenic twins. Respectively, anxiety and mood disorders were significantly more prevalent among the parents and siblings of probands with mood disorders. Schizophrenic spectrum disorders were significantly more common in the families of schizophrenic probands compared with relatives of mood disorder probands, thus confirming a relationship between schizophrenia and schizophrenic spectrum disorders. However, we cannot, based on our study, specify whether this relationship is caused by genetic or environmental factors.