BACKGROUND Profound hypercalcemia is usually due to underlying malignancy. METHODS We describe a case of granulomatous myositis presenting with extreme hypercalcemia of 20.1 mg/dl and generalized weakness that did not resolve despite rapid correction of serum calcium. The disease process was unmasked by cholecalciferol supplementation. Initial search for a malignant process yielded no diagnosis, but an elevated 1,25-dihydroxyvitamin D level, in the setting of a suppressed PTH and undetectable PTHrP, pointed to the presence of excessive 1α-hydroxylase activity. RESULTS Biopsy of the vastus lateralis muscle showed extensive granulomatous myositis. Immunohistochemical staining for 1α-hydroxylase was localized to the multinucleated giant cells and histiocytes. Musculoskeletal magnetic resonance imaging showed involvement of proximal muscle groups of both thighs and upper limbs. CONCLUSIONS Measurement of vitamin D metabolites is pivotal in diagnosing 1,25-dihydroxyvitamin D-mediated hypercalcemia. Granulomatous disease can occasionally cause profound hypercalcemia and needs to be considered in the differential diagnosis. 1,25-Dihydroxyvitamin D-mediated hypercalcemia is responsive to glucocorticoid therapy.