A 16-year-old girl was found to have coexistent polyostotic fibrous dysplasia and myositis ossificans progressiva. We postulate that both conditions result from a dysplastic connective tissue reaction involving bone in one instance and muscle in the other and leading to collagen deposition and metaplastic bone formation. Patients diagnosed as having either condition should be carefully scrutinized for evidence of the other to help determine whether this has been a chance occurrence or a more significant manifestation of a related fibrous and osseous metaplastic tissue transformation.