Seminomas are germ cell tumors that are rarely associated with hypercalcemia. In this report, four cases of seminoma with concomitant hypercalcemia are presented and another three from the literature are reviewed. All seven patients exhibited hypercalcemia with a normal serum concentration of inorganic phosphorus and no evidence of skeletal metastases. The peripheral venous level of parathyroid hormone (PTH) was normal in four of the five patients in whom it was measured. The serum concentration of calcitriol was elevated in the two patients in whom it was measured. After systemic chemotherapy, the serum "corrected" total calcium concentration returned to normal and remained normal; the decrease in the levels temporally paralleled the decrease in tumor volume. Both patients with elevated calcitriol levels remained eucalcemic after treatment of the malignancy, suggesting that the increased serum calcitriol level was linked to the development of hypercalcemia as this humoral agent was inappropriately elevated by patients with this syndrome. In contrast to many forms of malignancy, the development of hypercalcemia did not adversely affect the prognosis of the patients with seminoma, since all seven patients entered complete remission. Hypercalcemia appears to be heretofore unrecognized paraneoplastic syndrome associated with seminoma.