Gain-of-function mutations in fibroblast growth factor-23 (FGF23) are responsible for autosomal dominant hypophosphatemic rickets, a disorder of isolated renal phosphate wasting. Patients with the disorder display hypophosphatemia with normocalcemia as well as inappropriately normal 1,25-dihydroxyvitamin D [1,25(OH)₂D₃] concentrations. Reciprocally tumoral calcinosis (TC) patients are often hyperphosphatemic with inappropriately normal or elevated serum 1,25(OH)₂D₃ levels and have ectopic and vascular calcifications, a phenotype similar to that of Fgf23 null mice. Therefore, the goal of the present studies was to test whether FGF23 was a candidate gene for TC. Two sisters in a consanguineous TC family had hyperphosphatemia and normal 1,25(OH)₂D₃ levels with characteristic ectopic and vascular calcifications. Interestingly, these patients had low-normal intact serum FGF23 levels but demonstrated FGF23 concentrations approximately 40 times normal when assessed with a C-terminal FGF23 serum assay. Mutational analyses identified a homozygous S71G mutation in FGF23 in the TC patients, which was not found in control alleles. Finally, modeling demonstrated that the S71G mutation most likely destabilizes full-length FGF23. In summary, recessive FGF23 mutations can lead to TC. Additionally, our findings indicate that FGF23 may adopt an unstable conformation in some TC patients, possibly leading to nonfunctional FGF23 protein.