Background: Chronic hyperglycemia has been hypothesized to contribute to coronary heart disease (CHD), but the extent to which hemoglobin A 1c (HbA 1c) level, a marker of long-term glycemic control, is independently related to CHD risk is uncertain. Methods: We conducted a prospective case-cohort study of 1321 adults without diabetes and a cohort study of 1626 adults with diabetes from the Atherosclerosis Risk in Communities Study. Using proportional hazards models, we assessed the relation between HbA 1c level and incident CHD during 8 to 10 years of follow-up. Results: In adults with diabetes, the relative risk (RR) of CHD was 2.37 (95% confidence interval [CI], 1.50-3.72) for the highest quintile of HbA 1c level compared with the lowest after adjustment for CHD risk factors. In persons without diabetes, the adjusted RR of CHD in the highest quintile of HbA 1c level was 1.41 (95% CI, 0.90-2.30); however, there was evidence of a nonlinear relationship in this group. In nondiabetic adults, HbA 1c level was not related to CHD risk below a level of 4.6% but was significantly related to risk above that level (P<.001). In diabetic adults, the risk of CHD increased throughout the range of HbA 1c levels. In the adjusted model, the RR of CHD for a 1-percentage point increase in HbA 1c level was 2.36 (95% CI, 1.43-3.90) in persons without diabetes but with an HbA 1c level greater than 4.6%. In diabetic adults, the RR was 1.14 (95% CI, 1.07-1.21) per 1-percentage point increase in HbA 1c across the full range of HbA 1c values. Conclusion: Elevated HbA 1c level is an independent risk factor for CHD in persons with and without diabetes.