Magnesium, phosphate, immunoreactive parathyroid hormone and calcium were measured in the serum of nine patients with medullary carcinoma of the thyroid and high serum calcitonin, and compared with the values in nine patients with normal serum calcitonin. Serum phosphate was significantly higher in the group of patients with high serum calcitonin (P less than 0.025). No correlation was observed between the serum concentrations of magnesium and calcitonin, nor of immunoreactive parathyroid hormone and calcitonin.