Masked mastoiditis defines a subclinical infectious inflammatory process of the mucosal lining and the bony structure of the mastoid air cells, with intact tympanic membrane. It follows an apparently well-treated recent acute otitis media. Because of obstruction of the tympanic diaphragm by mucosal swelling, polypoid mucosa or granulation tissue, a complete separation of the tubo-tympanic cavity from the mastoid air cell system results. The latter remains unaereated and the local infectious mucosal disease progresses to osteitis. Probably due to colonizing flora in this unventilated media, the developing bone infection is 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 within the mastoid. Since this disease is characterized not by exudative but by proliferative changes, there is no pus formation. The incidence of complications is high. Plain x-ray and CT scan do not specifically define the disease process, but bone scan indicates the bone-invading nature of the mastoid infection. The osteoblastic reaction secondary to osteitis is demonstrated by high uptake of the isotope in the involved mastoid. Antibiotics may cure the disease, but in most cases surgery is unavoidable.