The Inventory to Diagnose Depression (IDD) is a self-report scale designed to diagnose DSM-III major depressive disorder (MDD). In our analysis, its test-retest reliability and internal consistency were high. The IDD was significantly associated with other self-report and interviewer rated depression scales and was sensitive to clinical change. Diagnostic agreement between the IDD and clinician's diagnosis of MDD was as high as that found in studies examining the interrater reliability of the diagnosis of MDD. Moreover, our results suggested that the IDD may aid clinicians in detecting secondary depression and distinguishing psychotic depression from nonaffective psychoses. The IDD may be particularly useful in light of the recent evidence that American psychiatrists continue to underdiagnose depression and overdiagnose schizophrenia.