Tumor-induced (oncogenic) osteomalacia is a rare clinicopathologic entity in which the clinical signs and symptoms of osteomalacia and the specific laboratory abnormalities of hypophosphatemia, hyperphosphaturia, and low serum levels of 1,25(OH)2 vitamin D are associated with the finding of a neoplastic process in the patient. To date, less than 100 cases of oncogenic osteomalacia have been described. We report a new case of adult-onset hypophosphatemic osteomalacia leading to the discovery of an asymptomatic phosphaturic mesenchymal lung tumor. Complete resection of the pulmonary neoplasia was followed by rapid normalization of the laboratory findings and clinical remission. The clinical, laboratory, and histopathologic spectrum of tumor-induced osteomalacia is presented, and the postulated mechanism of this condition is discussed in light of the relevant literature. The presence of occult neoplasms should be considered in cases of unexplained adult osteomalacia, with the physician's efforts being rewarded by the dramatic cure that follows excision of the tumor.