Florid reactive periostitis is a nonneoplastic reactive process that originates in the periostium. Usually, it arises on the short tubular bones of the hands and presents as a tender, rapidly growing mass: partial mineralization may be seen on radiographic examination. Histologically, it is composed of an admixture of reactive, hypercellular fibrous tissue and woven bone that has a trabecular pattern. Some cases have a cap of reactive cartilage on the surface. Osteosarcoma is the most important tumor to exclude in the histologic differential diagnosis. Simple excision is the treatment of choice. The prognosis is excellent; few cases recur.