We reviewed roentgenograms and clinical records in order to characterize the spinal deformity in forty patients who had an established diagnosis of fibrodysplasia ossificans progressiva. Twenty-six (65 per cent) of the patients had scoliosis, which, according to the clinical records and the recollection of the patients, had been present during childhood. Twenty-three (88 per cent) of the twenty-six curves were unbalanced c-shaped curves, while the remaining three (12 per cent) were balanced s-shaped curves. Twenty-one (91 per cent) of the twenty-three c-shaped curves involved the thoraco-lumbar or lumbar spine. The c-shaped curves ranged in magnitude from 15 to more than 80 degrees. Curves became rigid by early adulthood and many resulted in severe pelvic obliquity with impaired sitting or standing balance. An osseous bridge developed between the posterolateral aspect of the iliac crest and the posterolateral aspect of the rib cage in twenty-two (55 per cent) of the forty patients. Nineteen (86 per cent) of these twenty-two patients had scoliosis; there was a significant association between the development of scoliosis and the presence of the osseous bridge (p < 0.005). Ossification of the paravertebral muscles and fascia during the first decade of life limited the development of a normal thoracic kyphosis in ten (42 per cent) of twenty-four patients for whom lateral roentgenograms of the spine were available. A spinal orthosis was used to treat the scoliosis in two patients, but this method resulted in breakdown of the skin and failed to halt progression of the curve.(ABSTRACT TRUNCATED AT 250 WORDS)