Summary.Hypercalcaemia complicating acute myelogenous leukaemia is a rare but well‐recognized phenomenon. In most cases the pathogenetic mechanism causing the hypercalcaemia remains poorly understood. We recently studied in detail two patients with acute myelogenous leukaemia who developed hypercalcaemia during the course of their illness. The results of these studies conclusively excluded primary hyperparathyroidism or ectopic parathyroid hormone production as causes of the patients’ hypercalcaemia.In vitrostudies carried out on short‐term suspension cultures of one patient's peripheral blood blast cells demonstrated production of a factor with potent bone resorbing activity, distinct from parathyroid hormone (iPTH) and prostaglandin E2(PGE2). Further characterization of the bone resorbing factor suggested that it bore some similarity to osteoclast activating factor (OAF). Hypercalcaemia in the other case appeared to be due to a combination of skeletal invasion by malignant cells, and to ectopic secretion of an unidentified humoral factor with bone resorbing activity. These two cases demonstrate that the hypercalcaemia complicating acute myelogenous leukaemia may be due to a variety of mechanisms distinct from parathyroid hormone produ
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