Fibrodysplasia ossifcans progressive (FOP) is a rare genetic disorder characterized by congenital malformation of the great toes and by progressive heterotopic ossification of soft tissues. Although ankylosis of the temporomandibular joint occurs commonly in the late stages of the disease, only one well-documented case of submandibular heterotopic ossification in a patient who had FOP exists. Twelve (11 %) of our 107 patients who have FOP had submandibular heterotopic ossification that was mistaken initially in seven of the patients for mumps, angioneurotic edema, abscess, mononucleosis, or neoplasm. Two male patients and 40 female patients ranging in age from 6 to 47 years (mean, 21 years) were studied. Ten patients survived following assiduous precautionary measures. One patient who required emergency tracheostomy and ventilatory support also survived. Another patient died of inanition from chronic swallowing difficulty. An effective treatment program includes early identification of the submandibular flare-up, nutritional support, and glucocorticoid therapy. Submandibular swelling in patients who have FOP can be a medical emergency and requires intensive precautionary measures to avoid catastrophic clinical deterioration. These measures include avoidance of lesional manipulation, airway monitoring, and aspiration precautions. Submandibular swelling should be recognized as a variable feature of FOP.