We have found bioassayable somatomedin activity to be subnormal in 20 of 32 children and adults with β-thalassemia. The levels were comparable to values reported in growth hormone-deficient subjects. Since patients with thalassemia are not growth hormone deficient, the data suggest the possibility of defective hepatic biosynthesis of somatomedin. Increased iron stores in these patients, who have secondary hemosiderosis of many organs, including the liver, may depress somatomedin activity. Therapy for one year with daily subcutaneous infusions of the iron-chelating agent deferoxamine had no effect on mean bioassayable serum somatomedin activity.