Purpose Given that preoperative hyperglycemia is associated with poor outcomes and many non-diabetic patients have high plasma glucose (PG) levels, the purpose of our study was to estimate the prevalence of undiagnosed diabetes among non-cardiac surgery patients and to identify predictors of hyperglycemia in non-diabetics. Methods We included all non-cardiac surgery patients with complete records in the Clinical Database of the Anesthesiology Institute at the Cleveland Clinic during January 2007 to April 2009, and we estimated the prevalence of undiagnosed diabetes and impaired fasting glucose (IFG) among the non-diabetic patients. The mean glucose levels for known diabetics and undiagnosed diabetics were compared using two-tailed Student's t tests, and we assessed the association between PG levels and demographic variables within the non-diabetics. Results Of the 39,434 patients analyzed, 5,511 (14%) were known diabetics. Of the 33,923 known non-diabetics, 3,426 (10 %) were undiagnosed diabetics and another 3,549 (11%) had IFG. Thus, 6,975 patients (21%) of the non-diabetic patients presented with abnormally high glucose. Previously undiagnosed diabetics had higher preoperative glucose levels compared with known diabetics, with a mean ± standard deviation (SD) of 161±48 vs 146±67 mg-dL-1 (8.9±2.7 vs 8.1 ± 3.7 mmoL-L -1), respectively. The difference remained highly significant after adjusting for body mass index, age, sex, and American Society of Anesthesiologists (ASA) physical status (P \ 0.001). Among non-diabetics, older age, obesity, male sex, and a higher ASA physical status were collectively significant predictors of hyperglycemia, with a c-statistic (95% confidence interval) of 0.67 (0.66-0.68). Conclusion A significant proportion of non-cardiac surgery patients have previously undiagnosed diabetes and pre-diabetes. Previously undiagnosed patients have higher fasting glucose levels compared with diabetic patients. Further studies should be conducted to identify the implications of these findings on patient outcomes.