Introduction: Mental health concerns make up 5-10% of all adult presentations to Canadian emergency departments (ED). One challenge for the emergency physician (EP) is determining if a patient with a mental health concern has concomitant underlying medical illness. We defined “serious medical illness” (SMI) as a pathological condition that requires inpatient treatment on a medical or surgical ward. SMI undetected by emergency physicians in patients presenting with mental health concerns may result in adverse patient outcomes. The aim of this study was to determine the prevalence, timing, and etiology of undetected SMI in the ED among adult patients presenting with mental health concerns. Methods: A retrospective chart review was performed on all patients age 18 and older who presented to the ED at Victoria Hospital, London Health Sciences Centre between October 1, 2014 and April 30, 2015, who were subsequently referred to psychiatry by the EP. The primary outcome was the number of patients transferred to a medicine or surgery inpatient unit for treatment of their SMI within seven days of psychiatry admission from the ED. Results: 1,255 patients were referred to psychiatry during the study period. 803 patients were admitted and 452 were discharged. Of the admitted patients, 14/803 patients (1.7%) met our primary outcome. The mean age of patients in the SMI group (n=14) was 64 years. The mean age in the non-SMI group (n=1,241) was 38. In the SMI group, 3/14 patients died, 2/14 patients required an ICU admission, and 2/14 patients underwent a surgery for their missed SMI. The average length of psychiatry admission prior to transfer was 3.7 days. The average length of medical/surgical admission after transfer from psychiatry was 8.3 days. Undetected diagnoses included NSTEMI, serotonin syndrome, lithium toxicity, thoracic aortic aneurysm, gastrointestinal stromal tumour, forearm abscess, Parkinsonian crisis, and others. Conclusion: This chart review demonstrated a 1.7% rate of undetected serious medical illness in patients who presented to the ED with mental health concerns. Adverse outcomes included death, ICU admissions, and surgeries. This rate is similar to other studies on the topic. The SMI group tended to be older than the non-SMI group. This research may have implications on the appropriate workup and disposition of elderly patients presenting to the ED with mental health concerns.