The basal metabolic rate when accurately determined and its significance correctly appraised is of immense help both in diagnosis and in determining the progress and course of thyroid diseases. Over- or under-activity of the thyroid gland produces marked changes in the level of the basal metabolic rate because the secretion of the thyroid gland (the active principle of which is thyroxin) is a catalyst that accelerates the rate of heat production in proportion to its concentration in the body. It is not correct, however, to assume that all variations in the basal metabolic rate necessarily represent primary changes in the activity of the thyroid gland. For example, in leukemia the metabolism is usually markedly increased and in under-nutrition it is as a rule decreased. In certain cases of mental unrest or uneasiness the metabolism as obtained is considerably above the average normal, not because there is a technical error but because in that individual it is at first impossible to obtain the test under the standard condition of mental as well as physical rest, although after repeated determinations it is usually possible to obtain a basal rate and this will be found to be in most cases normal. It is impossible to do repeated determinations on all patients and it rests, therefore, on the clinical experience and acumen of the physician to select those cases for more intensive study which are likely to fall in this group. Unfortunately these patients simulate in some ways patients suffering from either adenomatous goiter with hyperthyroidism or exophthalmic goiter. Mistakes, therefore, can be easily made both in diagnosis and in judging the effect of treatment by a blind reliance on basal metabolic rate determinations. At the present moment radiologists are intent upon studying the effect of X-ray therapy on exophthalmic goiter. Frequently a decrease in the basal metabolic rate following such treatment is presented as evidence that there has been a corresponding clinical improvement. This is justifiable only if the determinations are technically correct and are truly representative of the basal or standard metabolism of that individual both before and after treatment.