BACKGROUND: Anemia still remains the most common single nutrient deficiency disorder in the world, especially among adolescence period because of the accelerated physical growth both in boys and girls and menstrual losses in iron and dieting for fear of obesity in female teenagers. The purpose of this study was to determine the prevalence of anemia and microcytosis among healthy students. METHODS: We conducted blood tests in apparently healthy girls (n=49,162) and 2,203 boys aged 10 to 18 years who went to school in Pucheon. Hemoglobin, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), white blood cell and platelet count were determined at same day with the use of a Coulter T 540 (Coulter Corp., F1, U.S.A.). RESULTS: 1) In boys the mean Hb concentration increased with age from 13.2g/dL in 10~12 yr to 15.3g/dL in 17~18 yr. In girls the mean Hb concentration was 13.3g/dL in 10~12 yr, 13.1g/dL in 15~16 yr, and 12.9g/dL in 17~18 yr. The difference in mean Hb levels between both sexes was significant (P<0.001) after 13 yrs of age. The mean MCV and MCH increased with age in each sex. 2) In boys, the prevalence of anemia showed 6.5% in 10~12 years, 2.4% in 13~14 years, 5.7% in 15~16 years and 3.0% in 17~18 years. In girls, the prevalence of anemia increased with age: 2.1% in 10~12 years, 4.2% in 13~14 years, 12.1% in 15~16 years and 20.5% in 17~18 years. 3) The prevalence of microcytosis (MCV< 78 fL in 10~14 yr; MCV< 79 fL in 15~18 yr) in boys was about 1% in each group. The prevalence of microcytosis in girls was2.0% in 10~12 years, 2.7% in 13~14 years, 6.1% in 15~16 years and 8.1% in 17~18 years. CONCLUSION: As the prevalence of anemia was substantially greater than expected, studies of iron deficiency in adolescent girls should be performed to assess iron deficiency. In addition, nutritional education for adolescents is warranted.