The cemento-ossifying fibroma (COF) is a mesodermal, non-odontogenic tumour of ectopic multipotential periodontal membrane blast cells. It is aggressive, locally destructive, and has a high recurrence rate. A case report of COF of the petromastoid region is presented. This location has not been described until now. Trauma may act as a trigger to sudden growth of the atopic periodontal tissue. Due to the aggressive behaviour of this tumour and its frequent recurrence radical surgery is needed.