BACKGROUND: In Korea, more than 20 cases of beta-thalassemia have been reported up to date. To detect -thalassemia. Hemoglobin (Hb) fractions were measured in patients with hypochromic microcytosis, and we analyzed the hematological characteristics of these patients. METHODS: Among 359, 369 CBCs performed at Asan Medical Center, 229 patients (0.064%) showed hypochromic microcytosis with less than 75 fL of mean corpuscular volume (MCV), less than 24 pg of mean corpuscular hemoglobin (MCH), and less than 18% of red cell distribution width (RDW). We analysed Hb fractions using high performance liquid chromatography (VARIANT(TM) Hemoglobin Testing System). Iron, total iron binding capacity (TIBC), ferritin, and reticulocyte counts were measured and medical history was searched on cases with Hb A2 and F fractions more than 3.5% and 2.0%, respectively. RESULTS: Among the 229 patients with hypochromic microcysis, 44 (19.2%) showed an increased level of Hb A2 and/or F fractions. With the exclusion of 28 patients (23 children <2 years old and 5 pregnant women), 16 (7.0%) showed a significantly increased level of Hb A2 and/or F. However, all 16 patients were diagnosed as having iron deficiency anemia based on their iron status and clinical findings. Three patients who had an increased level of Hb F at more than 5% needed a further study and follow-up to rule out the diagnosis of the hereditary persistence of the fetal hemoglobin. CONCLUSIONS: No thalassemia cases were found in the study. Incidence of beta-thalassemia should be very low, less than 1/359, 369 (0.00027%), in South Korea; a larger population should be screened to detect beta-thalassemia.