Morbidity risks for mental illness were determined in 750 first-degree relatives of chronic schizophrenic and normal control probands. Psychiatric disorders that were more frequent in relatives of schizophrenic probands than in relatives of normal control probands were chronic schizophrenia (5.8% versus 0.6%), schizotypal personality disorder (definite, 14.6% versus 2.1%; probable, 12.1% versus 6.5%), and paranoid personality disorder (7.3% versus 2.3%). The data suggest that schizotypal and paranoid personality disorders are genetically related to schizophrenia. The implications for schizophrenia research are discussed.