Purpose: Iron deficiency anemia (IDA) frequently occurs in adolescence because of accelerated physical growth in both boys and girls, and because of menstrual iron loss and dieting to avoid obesity in female teenagers. As little is known about iron deficiency in adolescents in Korea, this study was aimed at assessing the prevalence of iron deficiency and IDA in apparently healthy Korean adolescents in relation to gender, physical development, and dietary habit. Materials and Methods: With a questionnaire regarding their demographic characteristics, blood samples were obtained from apparently healthy students aged 10-18 years by venipuncture in September and October 1996. Hemoglobin (Hb), hematocrit, RBC indices, serum ferritin levels, iron and total iron-binding capacity (TIBC) were measured. Results: A total of 1,164 students (M:F = 610:554) aged 10-18 years were included in this study. In boys, the mean Hb concentration increased with age from 13.3 g/dl in the 10-12 years group to 15.3 g/dl in the 17-18 years group. In girls, the mean Hb concentration was 13.2 g/dl in the 10-12 years group, and 12.7 g/dl in the 15-16 years group. In boys, iron deficiency prevalence was 5.6% in the 10-12 years group, 10.6% in the 13-16 years. In girls, the prevalence of anemia increased with age: 0.8% in the 10-12 years group; 5.5% in the 13-14 years group; 16.9% in the 15-16 years group; 19.7% in the 17-18 years group. Iron deficiency prevalence in girls was 9.2% in the 10-12 years group; 14.1% in the 13-14 years group; 22.6% in the 15-16 years group; and 30.7% in the 17-18 years group. The prevalence of IDA was 0.8% in the 10-12 years group, and 13.1% in the 17-18 years group. According to the questionnaire, 19.5% of males and 40.0% of females usually missed one meal a day. The proportion of missed meals in girls increased with age. The proportions of Hb < 12 g/dl and ferritin < 10 ng/ml were significantly high in boys aged 10-14 years with vegetarian parents. The prevalence of anemia and iran deficiency in the students with menstruation was 14.9% and 21.6%, which was higher than the prevalence of 3.3% and 13.2% in those who did not have the menarche (P < 0.001 and P < 0.05, respectively). Conclusion: As the prevalence of iron deficiency and IDA was substantially greater than normal, studies of iron state in Korean adolescent girls, especially middle to high school age, should be undertaken to assess iron deficiency. In addition, nutritional education for adolescents is warranted.