OBJECTIVE: To report a case of fibromatosis in the submental region diagnosed at the Department of Otorhinolaryngology and Neck Surgery of Presidente Prudente/SP in December 2012. CASE REPORT: M. A. C., a 43-year-old woman mulatto, born in Panorama-SP presented at the Otorhinolaryngology service at Hospital Regional de Presidente Prudente-SP with a history of neck mass discovered 50 days prior associated with insidious and progressive growth along with dysphagia, trismus, and earache, but with no signs of inflammation. Her past medical history included fibrodysplasia ossificans progressive from birth associated with limited range of motion. Physical examination revealed lockjaw and an 8-cm hardened mass in the right submandibular region, which was painless on palpation. Tomography of the neck showed obliteration of cutaneous and deep planes of the bilateral submental region without delineation (fibrous tissue). Fibronasolaryngoscopy showed a bulging right oropharynx. Histopathologic examination of a biopsy specimen was consistent with fibromatosis. After reviewing the case, conservative treatment was recommended because of the extent of the injury, involvement of adjacent structures, and comorbidities of the patient. The patient continues with outpatient follow ups. CONCLUSION: This case demonstrates the difficulties in treating a rare disease such as submandibular fibromatosis and its complications in view of the importance of the affected area. Proposed treatments such as surgery, chemotherapy, and/or radiotherapy for the patient presented here were not feasible because of patient comorbidities, and a literature search failed to unearth any viable alternative treatments, further highlighting the challenge of managing this disease in patients with the comorbidities described here.