Exophthalmos usually did not appear nor did pre-existent exophthalmos progress in thyrotoxic patients treated with desiccated thyroid in full replacement dosages, 2½ to 3 grains per day, simultaneously with thyroid blocking agent therapy, ¹³¹I administration, or thyroidectomy. Indeed, in most patients so treated, exophthalmos present prior to therapy of the hyperthyroidism decreased or disappeared. Our observations are consonant with the hypothesis that full replacement dosages of thyroid hormones started at essentially the same time that treatment of the thyrotoxic state is initiated may provide prophylaxis against the appearance or the progression of the exophthalmos which follows treatment of thyrotoxicosis. We do not intend to suggest, however, that exophthalmos which appears, progresses, or enters a malignant phase after successful treatment of hyperthyroidism at some time in the past is benefited by such treatment.