A 9-year-old boy with X-linked hypophosphatemic rickets had a recurring oral giant cell lesion. These lesions are relatively uncommon in children and represent a potentially aggressive disorder that is microscopically indistinguishable from the brown tumors of hyperparathyroidism. Subclinical hyperparathyroidism is not uncommon in X-linked hypophosphatemic rickets and may account for the giant cell lesion in this patient.