A radioreceptor assay for serum 1,25-dihydroxyvitamin D (calcitriol) was used to screen patients with hypercalcemia of malignancy. Three patients with non-Hodgkin's lymphoma and hypercalcemia (serum Ca, 12.0, 13.4, and 13.0 mg/dL, respectively) had increased serum calcitriol levels (56, 72, and 77 pg/mL, respectively; normal, < 50 pg/mL). Elevated levels of calcitriol, an active vitamin D metabolite, occurred in the presence of significant renal impairment (creatinine clearance, 8 to 19 mL/min) and relative parathyroid suppression (serum immunoreactive parathyroid hormone, 17 to 39 μL-eq/ mL; mean value in end-stage renal disease, 182 ± 39 μLeq/mL). Hypercalcemia and excessive serum calcitriol levels responded to glucocorticosteroid therapy. In two patients, the hypercalcemia and increased serum calcitriol level were related to a tumor, but not to the serum immunoreactive parathyroid hormone level. Fractional intestinal 47Ca absorption, measured in one patient, was increased (0.94; normal, < 0.61) and varied directly with the serum calcitriol level. No patient had evidence of sarcoidosis. Hypercalcemia associated with certain lymphomas may be caused by the increased synthesis of calcitriol by lymphoma cells.