Perioperatively, insulin to treat hyperglycemia is administered judiciously to minimize the risk of hypoglycemia. In patients with diabetes in whom preoperative blood glucose levels are on the low end of normal, hypoglycemia risk may be underestimated. This retrospective study enrolled subjects with presenting preoperative blood glucose values in these ranges: 70 to 89 mg/dL (low normal group) and above 249 mg/dL (hyperglycemia-treated group). These groups were compared for subsequent perioperative hypoglycemia development. Subjects in the low normal group (n = 308) were older (P < .001), had increased incidence of renal disease (P = .02), and more prevalent beta-blocker use (P = .02) than the hyperglycemia-treated subjects (n = 279). Accounting for differences between groups, the incidence of perioperative blood glucose levels below 70 mg/dL was greater in the low normal group than the hyperglycemia-treated group (17.2% vs 3.6%, P < .001). Of subjects whose blood glucose levels fell below 70 mg/dL, blood glucose levels dropped below 50 mg/dL in 40% of hyperglycemia-treated subjects and 4% of low normal subjects. Perioperative hypoglycemia was likelier to develop in patients with diabetes who presented preoperatively with low normal blood glucose values than in patients treated with insulin for presenting hyperglycemia.