Oncogenic or tumor-induced osteomalacia-rickets is a syndrome characterized by hypophosphatemia, renal phosphate wasting, and decreased serum 1,25-dihydroxyvitamin D₃ levels. The tumors secrete a phosphaturic substance that causes total body phosphate depletion, leading to osteomalacia or rickets. Although the tumors are histologically polymorphous, personal review of 16 tumors documented to cause this syndrome revealed four morphologic patterns. The first contained 10 unique-appearing, mixed connective tissue tumors having variably prominent vessels, osteoclastlike giant cells, focal microcystic changes, dystrophic calcification, osseous metaplasia, and/or poorly developed cartilagelike areas. With one exception, all tumors of this group occurred in soft tissue and were benign. The single malignant tumor originated in bone, recurred locally, and metastasized to lung. The remaining tumors occurred in bone and showed benign clinical behavior. They resembled tumors known to occur in bone, that is osteoblas-tomalike (3 tumors), nonossifying fibromalike (2 tumors), and ossifying fibromalike (1 tumor).