Patients with fibrodysplasia (myositis) ossificans progressiva (FOP) (n = 28) were studied for up to 24 years. All had characteristic short big toes potentially recognizable at birth; there were radiographic changes in the toes, thumbs, cervical spine and metaphyses of the long bones, including exostoses. Ossification in the large skeletal muscles began from birth to 16 years (mean age 4.6 years) initially in 25 patients in the neck and upper spinal muscles, and later around the hips, major joints and jaw. The rate and extent of disability was unrelated to the time of onset. There was no evidence that any form of treatment produced consistent benefit. Despite the unique combination of skeletal abnormalities and ectopic ossification, the first diagnosis in patients with FOP was often wrong and usually delayed after ectopic ossification began (mean 2.7 years, range 0–14). Except where presentation was unusual, such as progressive stiffness, this delay was mainly due to failure to recognize the significance of the abnormal toes. The most frequent erroneous histological diagnoses were soft tissue sarcoma or fibromatosis. This series emphasizes the usually incorrect initial diagnosis, the misinterpretation of the histology, the unpredictable prognosis and the failure of current treatment. Despite its extreme rarity, there is a need for wider knowledge of this condition both to avoid clinical errors and to stimulate research.