Oncogenic osteomalacia has fascinated physiology-minded physicians for decades. The traditional name for this peculiar disorder connotes its classification as a paraneoplastic phenomenon. Such a characterization is a bit off the mark, however, in that the involved “neoplasm” is often (but not always) of limited clinical significance apart from its causal role in the musculoskeletal disease. Tumors responsible for oncogenic osteomalacia are usually benign rather than invasive, whereas generalized, debilitating osteomalacia and rickets are the important clinical problems for the patient. The assay for the measurement of circulating levels of fibroblast growth factor 23 (FGF-23), the development of which is described . . .