We report a case of oncogenic osteomalacia associated with a phosphaturic mesenchymal tumor in a 31-year-old woman. She was presented with severe generalized bone and muscle pain and was restricted to bed. She lost 20 cm in height over the 8 years since she had first noticed a pain in her thigh. A walnut-sized, hard, soft tissue tumor was found very easily beside her lower molar teeth Radiologic examination revealed a remarkable decrease in bone density and multiple pathologic fractures of spine, femur and phalangeal bones. Severe hypophosphatemia, hyperphosphaturia, low plasma 1,25-dihydroxyvitamin D3 level and high plasma PTH level were disclosed at presentation. Histomorphometric examination revealed an extensive area of unmineralized osteoid and little mineralizing activity. A pharmacologic dose of 1 alpha-hydroxyvitamin D3 or or 1,25-dihydroxyvitamin D3 slightly increased the serum phosphate level and renal tubular reabsorption of phosphate, and slightly decreased plasma PTH level without any symptomatic improvement. Histologic examination of the tumor revealed a mixed connective tissue tumor that consisted of central woven bones and surrounding primitive spindle cells with prominent vascularities. After removal of the tumor, all biochemical, hormonal and radiologic abnormalities disappeared with remarkable symptomatic improvement.