Schizoaffective disorder (SA D/O), introduced in 1933 by Dr. Jacob Kasanin, represented a first, modest change in our concept about the diagnoses of psychotic patients away from the beliefs of E. Bleuler, i.e., that hallucinations and delusions define schizophrenia, and toward the recognition of a significant role for mood disorders. SA D/O established a connection between schizophrenia and mood disorders, traditionally considered mutually exclusive, a connection that has strengthened progressively toward the diagnostic unity of all three disorders. A basic tenet of medicine holds that if discrepant symptoms can be explained by one disease instead of two or more, it is likely there is only one disease. The scientific justification for SA D/O and schizophrenia as disorders distinct from a psychotic mood disorder has been questioned. The "schizo" prefix in SA D/O rests upon the presumption that the diagnostic symptoms for schizophrenia are disease specific. They are not, since patients with severe mood disorders can evince any or all of the "schizophrenic" symptoms. "Schizophrenic" symptoms mean "psychotic" and not any specific disease. These data and a very low interrater reliability for SA D/O suggest that the concepts of SA D/O and schizophrenia as valid diagnoses are flawed. Clinically SA D/O remains popular because it encompasses both schizophrenia and psychotic mood disorder when there is a diagnostic question. We present a review of the literature in table form based on an assignment of each article assigned to one of five categories that describe the possible relationships between SA D/O, schizophrenia and psychotic mood disorders. We conclude that the data overall are compatible with the hypothesis that a single disease, a mood disorder, with a broad spectrum of severity, rather than three different disorders, accounts for the functional psychoses.