Purpose <p>Diabetes mellitus (DM) is increasing in prevalence. Perioperative hyperglycemia is associated with an increased risk of adverse postoperative outcomes. We aimed to determine the prevalence of DM as well as the incidence of patients having a current hemoglobin A1C (HbA1C) and compare postoperative outcomes between patients with and without hyperglycemia.</p> Methods <p>We performed a retrospective cohort study of adult patients undergoing noncardiac surgery at the University of Alberta Hospital (Edmonton, AB, Canada) between November 2019 and December 2023. We identified DM retrospectively using International Classification of Diseases, Ninth and Tenth Revision (ICD-9 and ICD-10), diagnosis codes. We evaluated HbA1C (90 days preceding admission) and the highest random glucose level on the operative day. We examined the unadjusted association of hyperglycemia with in-hospital mortality, 30-day mortality, and length of stay. We then undertook propensity-score-matched analysis.</p> Results <p>In adult patients, 42,996 noncardiac surgeries were performed. Of these, 9,421 (22%) patients were identified as having DM. Random glucose was not reported for 16% of patients with DM, and it was ≥ 10.0 mmol·L⁻¹ in 52.0% of patients with DM. HbA1C was available for 49% of patients with DM. In-hospital and 30-day mortality were significantly greater when the highest random glucose was ≥ 10.0 mmol·L⁻¹. Propensity-score-matched analysis found that 30-day mortality was greater among patients with DM.</p> Conclusions <p>In this large study, the prevalence of DM among patients who underwent surgery exceeded previously reported estimates. Perioperative hyperglycemia was significantly associated with increased postoperative mortality. Blood glucose monitoring was not performed systematically in patients with DM, and many lacked an HbA1C measurement within 90 days of surgery.</p>