Fibrodysplasia ossificans progressiva, a rare genetic disabling disease characterized by heterotopic bone formation, is of special interest for general medicine since the bone morphogenetic proteins (especially BMP-4) involved in its pathogenesis are known to play a role in skeletal morphogenesis, and the gene antagonist to BMP-4 noggin might be useful in preventing lamellar bone formation. We present two cases with characteristic musculo-skeletal abnormalities and histopathological features (inflammatory infiltrates which destroyed muscle tissue replacing it by proliferating fibroblasts). In one patient due to high activity of fibroblasts, the histopathologic pattern appeared to be atypical and the lesion was recognized by a general pathologist as sarcoma. The other patient, due to the symmetrical induration of sternocleidomastoid muscles, was primarily recognized as scleredema. We stress the diagnostic importance of skeletal abnormalities (hallux valgus and others), and discuss differentiation from progressive osseous heteroplasia (POH) and congenital or acquired localized cutaneous and muscle ossifications which have a much better prognosis, as well as Albright's hereditary osteodystrophy, which differs by the presence of various systemic abnormalities. A study of FOP might provide an important clue to the genetic molecular mechanism of bone formation, development of heterotopic bone and a possible prevention by molecular manipulation with the gene responsible for bone morphogenetic proteins and their antagonists.