Haematological parameters were determined in 146 pregnant patients seen before 20 weeks' gestation. Eighteen per cent had both reduced serum ferritin and raised serum transferrin values without significant anaemia, suggesting covert iron-storage deficiency. One group then received 120 mg elemental iron daily and the other no iron supplementation. Repeat haematological assessment 20 weeks later indicated that in 48% of patients a significant decrease in haemoglobin concentration occurred if no iron was given. Supplementary iron prevented this decrease from occurring in patients with iron stores, but was insufficient to restore depleted stores or to prevent depletion of iron stores in 34% of patients with adequate stores. It is suggested that patients with depleted iron stores require therapeutic doses of iron, whereas those with adequate stores require little or no iron supplementation.