The changes in thyroid function associated with adrenal crisis were studied in patients who had undergone bilateral adrenalectomy. Following interruption of the cortisone therapy the thyroidal uptake of radioactive iodine rose, as did the PBI concentration in the blood. On resumption of the cortisone administration, in connection with adrenal crisis, the uptake of radioiodine and PBI fell again – the latter to levels significantly lower than those prior to withdrawal of the cortisone. No similar fall was observed in adrenalectomized patients when corresponding doses of cortisone were given without preceding cortisone deprivation. The causal relationship of changes in thyroid function to adrenal crisis is discussed.