For 2 weeks 27 patients with hypercalcemia received a standard oral treatment with (3-amino-1-hydroxypropylidene)-1,1-bisphosphonate (APD) as the sole agent. Results were grouped according to causes of hypercalcemia and compared with effects of APD in 13 normocalcemic patients with Paget's disease of bone and 7 with osteoporosis. In 12 hypercalcemic patients with osteolytic bone lesions and in the 20 normocalcemic patients, the mean serum calcium decreased to final levels that were subnormal and significantly lower than those obtained after treatment of 8 patients with primary hyperparathyroidism. In 3 patients with myeloma and in 4 tumor patients without bone lesions, serum calcium did not always decrease to the normal range. Implications of these observations for the mechanism of hypercalcemia are discussed.