• Schizoaffective disorder, traditionally classified under schizophrenia, recently tends to be subsumed under affective disorder. This article reports a study of 35 sib pairs, where each six was independently diagnosed as having schizophrenia (SC), affective disorder (AD), or schizoaffective disorder (SA). The observed numbers of same-diagnosis pairs (ADAD, SASA, SCSC) were compared with the numbers expected if the three disorders are genetically independent. The results showed a significant deficiency only in the observed number of SASA pairs, which suggests that schizophrenia and affective disorder are genetically distinct whereas schizoaffective disorder is not. To test whether schizoaffective disorder is a variant of affective disorder or schizophrenia, the observed number of ADSA and SASC pairs were compared against the expected numbers. No significant differences were found, which suggests that schizoaffective disorder is genetically heterogeneous, with at least two subtypes, one a variant of affective disorder, the other a variant of schizophrenia.