The classification of functional psychoses has traditionally been dichotomous with schizophrenia and manic-depressive disorder, which are considered separate entities. However, the psychiatric literature is replete with descriptions of psychoses with mixed features. A variety of names has been applied to these psychoses, including the term "schizo-affective." This article examines the schizo-affective states across a variety of dimensions, including the acute symptomatologic picture, response to lithium carbonate therapy, follow-up studies, family history, and genetics. While the term "schizo-affective," as commonly used, probably describes a heterogeneous group of psychoses, considerable evidence supports the hypothesis that at least a subgroup of these psychoses has a definite relationship to the major affective disorders.