The production of immunoreactive PTH by leukemic cells in vitro was recently reported by Zidar and associates. In our patient, the inappropriately high serum PTH level which was "within the normal range" is in fact typical of ectopic hyperparathyroidism. In 54 such patients Riggs and associates, 7 using the same radioimmunoassay system,'-' found a mean PTH level of only 25~Eq/ml, in contrast to that in primary hyperparathyroidism which was 392 ktlEq/ml. This difference for the same degree of hypercalcemia has been explained by the observation that in ectopic hyperparathyroidism there were less circulating carboxyl-terminal fragments which react specifically with the antiserum. Although leukemia as primary cause of hypercalcemia is rare, this diagnosis should be considered in the face of unexplained hypercalcemia.