Introduction: Patients with poorly controlled diabetes mellitus (DM) often visit the emergency department (ED) for management of hyperglycemia, diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS). Many of these patients have a “sentinel” ED visit for other medical conditions prior to their hyperglycemic visit, which may worsen their glucose control. The objective of this study was to describe the epidemiology and outcomes of patients presenting with a sentinel ED visit prior to their visit for a hyperglycemic emergency. Methods: This was a health records review of patients ≥18 years presenting to one of four tertiary care EDs (combined annual census 300,000) with a discharge diagnosis of DM, hyperglycemia, DKA or HHS in a one-year period. Visits for hypoglycemia were excluded. Trained research personnel collected data from medical records including demographics, clinical history and results of investigations. Electronic charts were reviewed to determine if the patient came to the ED within the prior 14 days of their index hyperglycemia visit, and the details and outcomes surrounding both visits. Descriptive statistics were used where appropriate to summarize the data. Results: From January-December 2014, 609 ED visits had a discharge diagnosis of hyperglycemia. Mean (SD) age was 50.4 (19.5) years, and 343 (56.3%) were male. 101/609 visitors (16.6%) had an ED presentation within the previous 14 days from their hyperglycemia visit. 71 (70.3%) of these were discharged from this initial visit and 49/71 (69.0%) were discharged either without their blood glucose checked or with an elevated blood glucose (>11.0 mmol/L). Of the sentinel visits, 58 (57.4%) were for hyperglycemia and 15 (14.9%) were for infection. Upon returning to the ED, 45/101 (44.6%) visitors were subsequently admitted for management of severe hyperglycemia, DKA or HHS. Conclusion: This unique ED-based study demonstrates that patients with DM presenting with hyperglycemia or infection often return and may ultimately require admission. Clinicians should be vigilant in checking blood glucose when these patients present to the ED and provide clear discharge instructions for follow-up and glucose management. Future research should focus on improving glycemic control in these patients in order to prevent further hyperglycemic emergencies from occurring.