Iron deficiency is the most common cause of anemia in adolescents and is of continuing concern in Japan. We have conducted screening for anemia in adolescents in the Tokyo area since 1966. Screening for the prevalence of anemia is important both for identifying populations at risk and determining the appropriate treatment for individual patients. The number of junior and senior high school students enrolled in the screening program was 793 in 1966, which increased to more than 70,000 by 1989. A normal hemoglobin (Hb) level was present in 90% of students before 1981, and in 98% of boys and 95% of girls in 1990. The percentage of girls with a normal Hb level has decreased gradually since 1991. The high incidence of mild hypochromic microcytic anemia suggests an increase in the prevalence of iron deficiency. Dieting may be the cause of this undesirable phenomenon in adolescents.