Oncogenic osteomalacia is a syndrome characterized by phosphaturia, hypophosphatemia, decreased 1,25-dihydroxyvitamin D level and specific signs and symptoms of osteomalacia. It is associated with the presence of neoplasm originated from mesenchyme. Until now, less than 100 cases of oncogenic osteomalacia have been reported. The pathophysiology of oncogenic osteomalacia has not been fully understood, but it has been suggested that a certain substance released by tumor may inhibit not only la-hydroxylase activity and reduce 1,25-dihydroxyvitamin D level in part, but directly inhibit reabsorption of phosphate. And then, reduced phosphaturia, hypophosphatemia and eventually osteomalacia develop. We report a case of osteosarcoma induced oncogenic osteomalacia detected by MRI in 59 year old woman. (J Kor Soc Endocrinol 14:401-409, 1999)