The radiographic manifestations of nine South African patients with fibrodysplasia ossificans progressiva (FOP) have been analysed in detail. In three of these individuals, a 10--15 year follow-up was available. The condition usually presents before the age of 5 years; in one infant, abnormal calcification was demonstrated at 9 months. The disorder is remorselessly progressive but the hands, diaphragm and viscera are spared. Ossification in ligaments at the site of their attachment to long bones produces a 'pseudo-exostosis' appearance. Tethering of the shafts of long bones and growth at the proximal metaphyses can result in dislocation of shoulder and hip joints. In the hip joint the sloping configuration of the acetabular roof is produced by normal growth at the proximal metaphyseal end of the tethered femur. Calcification in periarticular tissues, together with bony and fibrous ankylosis, contributed to joint fixation. A similar process of growth inhibition leads to disparity in size between the bodies of the cervical vertebrae and their posterior appendages. It is concluded that apart from the developmental abnormality in the great toes, the skeletal changes in FOP are the consequence of soft tissue ossification and that the condition is not a primary bone dysplasia.