Three patients developed periarticular heterotopic new bone formation during the first two months after a CNS disorder. In each patient warmth, swelling, and tenderness around one or two joints initially resembled severe arthritis. Diagnosis was aided by roentgenograms of the involved areas that showed the ossification. While no treatment has been clearly demonstrated to interrupt the ossification and resultant contracture, a course of cautious physical therapy and antiinflammatory medication is probably indicated.