Fifteen paraplegic patients were followed to determine the onset, course, and remissions of ectopic-bone formation. Ectopic-bone formation occurred in seven patients and was accompanied by elevations of the serum alkaline phosphatase. In the eight patients in whom ectopic-bone formation did not occur there were no significant elevations of the serum alkaline phosphatase. It is felt, therefore, that the serum alkaline phosphatase is a reliable sign of the onset and remission of ectopic-bone formation in paraplegic patients.