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A systematic review of the claims files of all 420 veteran twin pairs from the National Academy of Sciences/National Research Council twin panel of 15,909 pairs where one or both twins had a psychotic diagnosis revealed that the monozygotic pairwise concordance rate for schizo-affective disorder is more than two times higher than that of schizophrenia, but not significantly different from that of manic depressive illness. Monozygotic twins concordant for schizo-affective disorder had affective symptomatology equal to that of manic depressive twins and schizophrenic symptomatology equal to that of the schizophrenic twins. For both twins in MZ pairs concordant for illness, schizo-affective psychosis has a mean age of onset which is earlier than both manic depressive psychosis and schizophrenia. Seven of 21 (33%) MZ index schizo-affective twins committed suicide, as opposed to none of the 18 manic depressives and three of the 100 index schizophrenic twins. The results are discussed in terms of a possible pathogenesis of schizo-affective disorder.