THE COMPLICATIONS of radioactive iodine therapy of hyperthyroidism are few. A theoretically possible complication is the occurrence of radiation-induced leukemia. This has probably followed x-ray therapy of ankylosing spondylitis and after the high-dosage radioactive iodine therapy of thyroid cancer. In an attempt to answer the question whether the lower dosage of radioactive iodine that is used in treatment of hyperthyroidism may be incriminated in leukemogenesis, Pochin analyzed data from 60,000 patients and found 18 cases of leukemia occurring during the 222,000 patient years of follow-up. This figure compared with an estimated expected incidence of 21 cases that might have occurred by chance. Because the number of cases in both the "observed" and "expected" series is small, it would be desirable to extend such observations to a larger number of patients. If such a relationship does exist, it is obviously important to define as early as possible the magnitude of the