Hypercalcemia of thyrotoxicosis is sometimes severe and may mask other symptoms of the disease. A case is presented in which hyperparathyroid crisis was initially suspected. Thyrotoxicosis was confirmed on emergency indication by 131I-uptake test and later by hormone assays. Forced diuresis and prednisolone therapy failed to lower the serum calcium level, but a dramatic effect was obtained with calcitonin. After 10 days the patient was normocalcemic on treatment with only antithyroid drugs. At thyroid resection, five months later, four normal parathyroid glands were found.