Objective: While the global epidemic of diabetes mellitus in adults is well documented, there are few reliable estimates of the prevalence of diabetes and prediabetes in adolescents in the United States. We conducted systematic hemoglobin A1C (HbA1C) screening in a very large adolescent blood donor population. Methods: A screening program was implemented in which HbA1C was measured at the time of blood donation among 31,546 volunteer high-school blood donors, 16-19 years of age, who participated in school blood drives conducted by Carter BloodCare, the largest independent blood program in Texas, between 2011 and 2013. The main study measures were the prevalence of HbA1C in the prediabetes range, 5.7-6.4% (39-46 mmol/mol) and in the diabetes range, ≥ 6.5% (≥ 48 mmol/mol), in the overall cohort as well as stratified by sex, race/ethnicity, and sociodemographic characteristics. Results: In the overall cohort, the prevalence of HbA1C in the prediabetes range was 11.0% (95% CI 10.7, 11.4), and the prevalence of HbA1C in the diabetes range was 0.53% (95% CI, 0.46, 0.62). The prevalence of HbA1C ≥ 5.7% (including prediabetes and diabetes) was higher in boys compared with girls (15.7% vs.7.9%, P<0.001), and was especially high in race/ethnic minorities (32.7%, 19.7%, 13.1%, respectively in Blacks, Asians, Hispanics) compared with 8.0% in Whites (p<0.001 for each vs. Whites). In analyses of cardiovascular risk factors stratified by HbA1C categories, higher total cholesterol and systolic and diastolic blood pressure were observed in donors with HbA1C in the prediabetes and diabetes range (P trend < 0.001 for all, Table ). Conclusions: HbA1C values in the prediabetes or diabetes range are common among adolescent blood donors, particularly among boys and ethnic minorities. Blood donation programs can be used as a valuable portal for public health screening, and may provide the opportunity for targeted public health interventions, triaging of donors to medical care, and research programs for community-based participatory interventions.