Substantial evidence suggests that genetic factors contribute to the aetiology of both schizophrenia and alcoholism, when they occur alone. To examine the role of genetic factors in schizophrenia and alcoholism when they occur together in the same individual, the frequency of both conditions was investigated in the co-twins of 34 monozygotic (MZ) and 47 dizygotic (DZ) index twins with a diagnosis of both schizophrenia and alcoholism. Both disorders alone were significantly more common in the MZ than in the DZ co-twins, suggesting that individuals suffering from schizophrenia and alcoholism have a genetic predisposition to both disorders, which is of the same nature as that which causes the two when they occur alone. In the co-twins of the MZ index twins, the diagnoses of schizophrenia and alcoholism were uncorrelated, indicating that the specific environmental factors of causal importance in the two disorders are not closely related.