The term masked mastoiditis defines a subclinical infectious inflammatory process of the mucosal lining and bony structures of the mastoid air cells with an intact tympanic membrane. The disease follows an apparently well treated recent acute otitis media. Probably due to an anaerobic colonizing flora, the developing bone infection is of low grade without pus formation. The clinical features of the disease are not overt as those in coalescent mastoiditis. The intact ear drum does not reflect the severity of bone eroding disease which is characterized by non-exudative but proliferative changes. Hence there is no pus formation. The incidence of complications is high. Plain X-ray film and CT scan do not specifically define the disease process. Bone scan indicates the bone invading nature of the mastoid infection. The osteoblastic reaction secondary to osteitis is demonstrated by the high uptake of the isotope in the involved mastoid. Antibiotics may cure the disease process but in most of the cases surgery in unavoidable.