Study objective Among patients with type 2 diabetes treated with insulin, perioperative hyperglycemia and hypoglycemia may cause undesirable symptoms, surgery delay or cancellation, or unexpected hospitalization. Our objective was to compare preoperative glargine dosing regimens on perioperative glycemic control in patients undergoing ambulatory surgery. Design Observational study. Setting Pre- and postoperative holding areas. Patients One hundred fifty patients with type 2 diabetes using a once daily, evening insulin glargine regimen undergoing ambulatory surgery were included. Interventions None. Measurements To conduct the analysis, patients were divided into four groups based on the percentage of normal evening glargine dose taken. Group 1 took no glargine. Group 2 took 33%-57%. Group 3 took 60%-87% and Group 4 took 100% of their normal dose. The primary outcome was the proportion of patients in each group with blood glucose in the target range (100-180 mg/dL), and the incidence of hypoglycemia (defined as BG <70 mg/dL or symptomatic, requiring glucose). Main results Group 3 had the highest proportion (78%) of patients within target range (P < .001) and Group 4 had the highest proportion of patients with hypoglycemia (P = .01). Patients in Group 3 were significantly more likely to achieve target blood glucose than patients in either Group 1 (P = .001) or Group 4 (P = .002). Conclusions Our study shows that the percent of normal insulin dose given the evening before surgery directly impacts perioperative glucose levels in ambulatory surgery patients. Patients taking 60%-87% of their usual dose the evening before surgery were likely to arrive in target blood glucose range with decreased risk for hypoglycemia. The mean and mode dose taken in Group 3 were 73% and 75%, respectively, suggesting that the optimal dose may be 75% of normal dose.