Haematological values of 35 pregnant women with beta-thalassaemia trait were followed during pregnancy. The discriminant function, calculated from haematological indices, was of no value in diagnosing beta-thalassaemia trait during pregnancy. Initially patients were given iron supplements only if the serum iron and total iron binding capacity levels indicated iron deficiency, but bone marrow biopsies performed in the first 22 patients at 32 weeks indicated deficient iron stores. These patients were therefore given iron irrespective of their serum iron level. All subsequent patients with beta-thalassaemia were also put on iron routinely at booking. Retrospectively the patients were divided into two groups. Patients in group 1 (18 patients) had received iron for less than 12 weeks, and their haemoglobin levels fell significantly during pregnancy (P less than 0-001). Haemoglobin levels in 16 patients who had received iron for more than 12 weeks (group 2), however, did not fall significantly during pregnancy (P less than 0-6). It is suggested (contrary to common practice) that patients with beta-thalassaemia trait should be given iron supplements during pregnancy. Serum folate and vitamin B12 levels did not change significantly in these patients and there was no increase in the incidence of maternal or fetal complications.