A 42-year-old man presenting with symptomatic hypercalcemia was successfully treated with corticosteroids. Initially he was thought to suffer from Addison's disease. A thyrotoxic state was, however, disclosed during the treatment. Evidence suggests that the hypercalcemia was caused by thyrotoxicosis. The effects of thyroid and adrenocortical hormones on calcium metabolism are discussed. Corticosteroids seem valuable in differentiating thyrotoxic hypercalcemia from coincidental hyperparathyroidism.