We describe the case of a middle-aged man who presented with manifestations of acute mastoiditis caused by fibromatosis of the mastoid region. A lesion of the right mastoid bone had eroded its wall and extended toward the middle and posterior cranial fossae. The macroscopic and microscopic appearance of an excised portion of the lesion established the diagnosis of mastoid fibromatosis. After a more detailed work-up, a second procedure involving extensive removal of the tumor was performed, and the diagnosis was confirmed. The patient's postoperative period was uneventful, and he showed no evidence of recurrence during 3 years of follow-up.