In this report from the Rhode Island Methods to Improve Diagnostic Assessment and Services (MIDAS) project, we describe the development and validation of the Diagnostic Inventory for Depression (DID), a new self-report scale designed to assess the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV; American Psychiatric Association, 1994) symptom inclusion criteria for a major depressive episode, assess psychosocial impairment due to depression, and evaluate subjective quality of life. A large sample of 626 psychiatric outpatients completed the DID and were interviewed with the Structured Clinical Interview for DSM-IV (SCID). The measure's test-retest reliability, discriminant and convergent validity, and sensitivity to clinical change were investigated. The DID subscales achieved high levels of internal consistency and test-retest reliability. The DID was more highly correlated with another self-report measure of depression than with measures of anxiety, substance use problems, eating disorders, and somatization, thereby supporting the convergent and discriminant validity of the scale. The DID also was highly correlated with interviewer ratings of the severity of depression and psychosocial functioning, and DID symptom severity scores were significantly different in depressed patients with mild, moderate, and severe levels of depression. The DID was a valid measure of symptom change. Finally, the DID was significantly associated with a diagnosis of major depressive disorder.