A basic view 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 bipolar disorder (BD) and schizophrenia (SZ) as distinctive disorders has been questioned. The diagnosis of schizophrenia rests upon the presupposition that the diagnostic symptoms are disease specific. They are not, since patients with severe mood disorders can demonstrate any or all of the schizophrenic symptoms. Furthermore, there is consistent evidence that genes contribute to the etiology of psychosis. Recent findings from genetic studies provide evidence for an overlap in genetic susceptibility across the traditional psychosis categories. Candidate genes show strong associations with component symptom complexes, such as psychosis, that are not projected directly onto Kraepelinian disease entities. In this paper we will review the literature that describes the possible relationships between bipolar disorder and schizophrenia.