Objective: Oncogenic osteomalacia is a paraneoplastic syndrome characterized by severe hypophosphatemia, hyperphosphaturia, and osteomalacia subordinated to renal loss of phosphate. It is caused by overproduction of phosphatonin fibroblast growth factor-23 by benign mesenchymal tumors which, when surgically eliminated, returns to basal levels. Methods: We report a case of a patient with oncogenic osteomalacia from a sacrum mesenchymal tumor whose FGF-23 levels remain elevated two years after successful surgical treatment. Results: A 53-year-old man with severe difficulty to walk and stand, severe hypophosphatemia, hyperphosphaturia, and raised levels of alkaline phosphatase and FGF-23 was diagnosed with oncogenic osteomalacia after a thorough process of elimination of causes of hyphosphatemia were excluded. Despite systemic chemotherapy, surgical removal of sacrum, primary mesenchymal tumor, phosphate and calcium supplementation, and no evidence of tumor recurrence, the serum concentration of FGF-23 remains elevated two years after discharge. Conclusions: The possible underlying mechanisms of sustained FGF-23 serum concentrations in this patient suggest the presence of molecular alterations in the FGF/FGFR signaling pathway.