Hemoglobin (Hb) Hiroshima is an Hb variant that travels rapidly on electrophoresis and shows a fourfold increase in oxygen affinity and a decreased Bohr effect. We encountered a 40-year-old male patient with erythremia and an undetectable HbA(1c) level. The presence of an abnormal hemoglobin molecule was suggested by the results of high-performance liquid chromatography analysis. Subsequent gene analysis by direct sequencing confirmed Hb Hiroshima (β146 histidine → aspartic acid). Caution should be exercised when diagnosing erythremia.