Tumor-induced osteomalacia (TIO) is usually caused by humoral factors that are secreted by a phosphaturic mesenchymal tumor. Fibroblast growth factor 23 (FGF-23) is predominantly produced by osteocytes in a normal physiologic state, but is elevated in many cases of TIO. FGF-23 inhibits phosphate reabsorption in the proximal renal tubule and also inhibits 1α-hydroxylase, resulting in decreased 1,25-dihydroxyvitamin D3 (1,25[OH]2D3) levels. We describe a patient who presented with symptoms of osteomalacia and was found to have a phosphaturic mesenchymal tumor in the soft tissue of the foot. The patient was informed that data concerning the case would be submitted for publication, and he provided consent. A fifty-eight-year-old man presented with multiple episodes of insufficiency fractures and diffuse pain involving the knees, hips, and feet. The patient was well and physically active until two and one-half years prior, when he experienced severe pain in the left knee and leg after playing soccer. Despite physical therapy and corticosteroid injections, the patient continued to have severe pain that gradually limited weight-bearing and walking. Laboratory tests revealed low phosphorus (1.3 mg/dL; reference range, 2.5-4.5 mg/dL), low 1,25(OH)2D3 (12 pg/mL; reference range, 18-72 pg/mL), and elevated alkaline phosphatase (260 U/L; reference range, 38-126 U/L). Rheumatologic workup, including erythrocyte sedimentation rate, antinuclear antibody, and rheumatoid factor, was negative. Radiographs did not reveal obvious fractures, but magnetic resonance imaging (MRI) and whole body technetium-99m bone scintigraphy revealed findings consistent with multiple stress and insufficiency fractures involving the sixth and eleventh ribs, the right tibial plateau, the left calcaneus, the distal part of the tibia, the distal femoral condyles, the fibular head, and the lateral tibial condyle. Bone densitometry revealed a T score of –2.6 in the femoral neck region, consistent with osteoporosis. Prior to presentation to our office, the patient continued …