The CT appearance of ectopic bone and its maturation in 25 patients were correlated with the findings on radiographs and bone scans. Ossification progressed from an early appearance of soft-tissue density of lower attenuation than muscle to a calcific density paralleling radiographic and scintigraphic evidence of bone formation. Persistent unossified, low-density soft tissue was detected adjacent to mineralized areas of ectopic bone in 14 patients up to 16 years after neurologic injury, often with bone-scan evidence of maturity of the ectopic bone. This soft tissue most likely corresponds to immature, unossified connective tissue, which may have a potential for ossification. Detection of areas of soft-tissue density by CT and their avoidance during surgical resection of an ankylosing mass of ectopic bone may reduce intraoperative hemorrhage and postoperative ectopic bone recurrence.