BACKGROUND: Opportunistic disease screening is the routine, asymptomatic disease screening of patients at the time of a physician encounter for other reasons. While the prevalence of unrecognized diabetes in community populations is well known, the prevalence in clinical populations is unknown. OBJECTIVE: To describe the prevalence, predictors, and clinical severity of unrecognized diabetes among outpatients at a major medical center. DESIGN AND SETTING: A cross-sectional observational study at the Durham Veterans Affairs Medical Center. SUBJECTS: Outpatients without recognized diabetes (N = 1,253). METHODS: We screened patients for diabetes by using an initial random Hemoglobin A1c (HbA1c) measurement, and then obtaining follow-up fasting plasma glucose (FPG) for all subjects with HbA1c ≥6.0%. A case of unrecognized diabetes was defined as either HbA1c ≥7.0% or FPG ≥7 mmol/L (126 mg/dL). Height and weight were obtained for all subjects. We also obtained resting blood pressure, fasting lipids, and urine protein in subjects with HbA1c ≥6.0%. RESULTS: The prevalence of unrecognized diabetes was 4.5% (95% confidence interval [CI], 3.4 to 5.7). Factors associated with unrecognized diabetes were the diagnosis of hypertension (adjusted odds ratio [OR], 2.5; P =. 004), weight >120% of ideal (adjusted OR, 2.2; P =. 02), and history of a parent or sibling with diabetes (adjusted OR, 1.7; P =. 06). Having a primary care provider did not raise or lower the risk for unrecognized diabetes (P =. 73). Based on the new diagnosis, most patients (61%) found to have diabetes required a change in treatment either of their blood sugar or comorbid hypertension or hyperlipidemia in order to achieve targets recommended in published treatment guidelines. Patients reporting a primary care provider were no less likely to require a change in treatment (P =. 20). CONCLUSIONS: If diabetes screening is an effective intervention, opportunistic screening for diabetes may be the preferred method for screening, because there is substantial potential for case-finding in a medical center outpatient setting. A majority of patients with diabetes diagnosed at opportunistic screening will require a change in treatment of blood sugar, blood pressure, or lipids to receive optimal care.