Tumor-induced osteomalacia is a rare acquired metabolic disorder characterized by hypophosphatemia and inappropriately low serum levels of 1,25-dihydroxyvitamin D. Symptoms include chronic muscle and bone pain, weakness, and fatigue in association with a high risk of fragility fractures due to osteomalacia. The diagnosis is commonly delayed for years due to the nonspecific nature of the presenting symptoms, failure to include determination of serum phosphorus levels in blood chemistry testing, and difficulty in identifying the responsible tumor. The pathogenesis of tumor-induced osteomalacia involves tumor expression of fibroblast growth factor 23, a hormone that inhibits proximal renal tubular reabsorption of phosphate and down-regulates renal conversion of 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D. The metabolic abnormalities may be partially or completely corrected with phosphate supplementation and calcitriol. A definitive diagnosis and treatment require excision of the responsible tumor.