The frequency of iron-deficiency anaemia was investigated in 376 children with the aid of serum ferritin determinations. The children, resident in Budapest or Miskolc were 6 months to 3 years old. Marked iron-deficiency (serum ferritin less than 10 micrograms/l) could be demonstrated in 33.8% of the investigated population. The frequency of iron-deficiency was related to the socio-economic and cultural status of the mother, a factor which is thought to influence the feeding habits. In areas where iron-deficiency is frequently detected regular iron-prophylaxis is recommended.