Since the work of the Mackenzies1, 2 and of Astwood and others3, 4 the use of thiouracil and related drugs has effected an important advance in the study of thyroid disease and may bring about a permanent change in the principles of their clinical management. Many cases have been reported, but opinions about the usefulness of these drugs vary widely. Enthusiastic observers tend to believe that thyroidectomy for hyperthyroidism will be replaced by medical treatment5. Others recommend the preoperative use of thiouracil for more complete control of hyperthyroidism6. Still others believe that the drug is dangerous and that compared with the well proved value of iodine and surgery, its disadvantages outweigh its benefits. Whatever the final decision may be, it should be kept in mind for the present that a definite mortality rate, probably somewhat over 0.5 per cent, is directly connected with the use of the drug and is entirely apart from the mortality rate of hyperthyroidism itself. It seems likely that this risk superimposed on the risk of the disease exceeds the mortality rate with treatment with iodine and competent surgery. It is possible that some of the newer related drugs, such as thiobarbital, are less toxic. It has been suggested recently7 that vitamin B6 (pyridoxine) may be useful in the treatment of leukopenia of toxic origin such as may occur in the course of thiouracil therapy. Many important questions remain to be answered, among which are whether or not treatment with thiouracil will be followed by a. . .