Clinician-rated measures are used in clinical trials and measurement-based clinical care settings to assess baseline symptoms and treatment outcomes of major depressive disorder (MDD), with a widely held dictum that they are sufficient in assessing the patient's clinical status. In this study, we examined clinician-rated measures of depressive and global symptom severity, obtained by interviewing patients as well as informants in an attempt to examine the potential difference or similarity between these two sources of information. The sample consisted of 89 treatment seeking, DSM-IV diagnosed MDD outpatients treated between 1995 and 2004. The clinician-rated measures used included the Montgomery Åsberg Depression Rating Scale (MADRS), and the Clinical Global Impression Scale (CGI) for Severity. The scores of the clinician-rated measures collected from patients' interviews were compared with those collected from informants' interviews. Clinician-rated scores, collected by interviewing patients, were significantly higher and indicative of greater symptom severity when compared with those collected by interviewing informants. This was true for both the MADRS before (P<0.0001) and after treatment scores (P<0.0001), as well as the CGI before (P<0.0001) and after treatment scores (P<0.0001). Consistently involving informants and the time/burden it takes for them to participate might not be practical in MDD clinical trials or everyday clinical care. The discrepancies observed between the clinician-rated scores obtained from patients and informants emphasize the importance of incorporating collateral information during the assessment and rating of depressive symptom severity in both clinical trials as well as in clinical practice.