In a retrospective analysis, the course, symptoms, treatment response, and personality of 54 delusional and 66 nondelusional unipolar depressed patients were compared. The delusional patients had more guilt feelings and were more ruminative, agitated, and referential than the nondelusional patients. They had a poor response to a tricyclic antidepressant therapy but good treatment outcome with a tricyclic-antipsychotic combination or ECT. The form and content of prior episodes were remarkably similar to the index episode in both groups. The authors believe that these findings support the conception of unipolar delusional depression as a distinct subtype of depressive illness.