Background: The Mood Disorders Questionnaire (MDQ) is a widely used screening instrument for bipolar disorders. The MDQ has seldom been used in the inpatient setting, and we report a clinical, real-world inpatient validation. Methods: Between April 2011 and August 2013, patients admitted to the inpatient Mood Disorders Unit completed an MDQ as part of their admission process. Patients with a discharge diagnosis of unipolar or bipolar disorders were included. The sensitivity and specificity were calculated for each number of questionnaire items checked positive, as well as the symptoms clustered around the same time and with moderate impairment in functioning. Results: A total of 1330 patient MDQ's were identified, and after excluding incomplete MDQ's and non-unipolar or bipolar diagnoses (e.g. anxiety, adjustment, or schizoaffective diagnoses), 860 MDQ's remained. One hundred fifty four patients (18%) were diagnosed with bipolar disorder, and 706 (82%) with unipolar depressive disorder. The average length of stay was 7.6 days. The optimal cutoff score was 8, resulting in a sensitivity/specificity of 86%/71%, compared to 92%/64% with a cutoff of 7. Limitations: Retrospective study using clinical diagnoses instead of research instrument diagnoses. Conclusions: The sensitivity of the MDQ in an inpatient mood disorders setting was higher than an outpatient psychiatric population, but the specificity was lower. A cutoff of 8 instead of the recommended outpatient cutoff of 7 was optimal. In today's busy clinical practices, a screening instrument for bipolar disorder is still useful, and the MDQ can be effectively utilized on an inpatient psychiatry mood disorders unit.