Eleven years has now elapsed since Plummer1introduced the use of iodine in the management of toxic goiter. During that entire period, iodine has been given as a routine in the Thyroid Clinic of the Massachusetts General Hospital and in some 575 cases its action has been made a matter of careful study. It is our purpose at the present time to evaluate this whole experience and to state the conclusions derived from it in the form that seems soundest to us today. We believe this a useful undertaking because it is evident that much confusion exists even now in the professional mind as to what iodine will and won't do in thyrotoxicosis. A brief summary of the biologic facts of the problem is in order. First of all, it may be stated that, so far as is known, the sole function of the thyroid is the manufacture of