In order to develop an objective test for discriminating between patients with thalassaemia intermedia requiring blood transfusion, and those not likely to require transfusion the medullary width (MW) in the midpoint of the second left metacarpal and the bone mass were measured in 34 normal children and in 37 patients. In patients, blood haemoglobin (Hb) and serum ferritin concentration were measured and cephalofacial deformities (CFD) were scored. The mean values of MW were 0.37 +/- 0.06 and 0.44 +/- 0.1 cm (P less than 0.01) and the bone mass 0.091 +/- 0.012 and 0.078 +/- 0.02 (P less than 0.005) in normal children and patients, respectively. In 13 of these patients who had MW more than 2 SD above the mean of the controls, i.e. more than 0.5 cm, regular blood transfusions were instituted. Measurements of MW 12 and 1 month before and 12 months after the initiation of transfusions showed an increase from 0.50 to 0.60 and a decrease to 0.49, respectively. Bone mass measured at the same times decreased from 0.083 to 0.045 and increased to 0.071, respectively. These changes were visible radiologically. It is concluded, therefore, that the measurement of MW seems to be an objective, simple test for discriminating between patients requiring or not blood transfusions, and that bone deformities will be reversible if transfusions are instituted using as criterion the MW (greater than 0.5 cm) regardless of age or haemoglobin concentration. This test may help clinicians to decide about the optimal time for institution of regular transfusions in patients with thalassaemia intermedia.