Hemoglobin A1c (HbA1c) and glycated albumin (GA) are glycemic control status indicators in patients with diabetes mellitus. Hemoglobin H (HbH) disease is a moderately severe form of α-thalassemia. Here we examine the usefulness of HbA1c and GA in monitoring glycemic control in patients with HbH disease. HbA1c, GA, and an oral glucose tolerance test were performed in 85 patients with HbH disease and 130 healthy adults. HbA1c was measured using five methods, including two systems based on cation-exchange high-performance liquid chromatography (Variant II Turbo 2.0 and Bio-Rad D100), a capillary zone electrophoresis method (Capillarys 3 TERA), a boronate affinity HPLC method (Premier Hb9210), and an immunoassay (Cobas c501). Significant lower levels of HbA1c were observed in patients with HbH disease than in healthy adults. In contrast, GA showed no statistically significant differences between participants with and without HbH disease. A considerable number of diabetic patients with HbH disease would be missed if using HbA1c as a diagnostic criterion for diabetes mellitus. GA but not HbA1c is suitable for monitoring glycemic control in patients with HbH disease that can modify the discriminative ability of HbA1c for diagnosing diabetes.