Summary. Serum iron, serum iron-binding capacity, serum ferritin and erythrocyte protoporphyrin were determined during uncomplicated pregnancy in 45 healthy women; 22 were given oral iron while the others were given a placebo. When iron was not given, 15 out of 23 women had exhausted iron stores and iron deficiency at term, as judged from low serum ferritin, low serum transferrin saturation and high erythrocyte protoporphyrin values. Only seven of them had a haemoglobin concentration between 10 and 11 g/dl at term but none had values < 10 g/dl. In the iron-treated group (n=22) none of the women developed iron deficiency. Serum ferritin was the most sensitive and specific test of iron deficiency. A practical procedure to detect iron deficiency and to control iron supplementation in pregnancy is suggested.