The present study reports a case in which a clinically asymptomatic radiolucent change surrounded by a sclerotic wall developed below the second molar in the mandible of a middle-aged man. The change was diagnosed as a typical case of Stafne bone cavity. In the earlier literature, development of a mandibular bone cavity has been described in only three patients. The appearance of bone cavities in a mandible that has previously been found normal indicates that these defects are not congenital but developmental processes.