From personnel subjected to the Submarine Escape Training Tank at New London, a group was selected at random consisting of 90 men who were judged to have aero-otitis media of the severest, or No. 4, grade according to Dr. Teed's categories of otopathologic change. A brief description of the No. 4 category is given. Post-pressure audiograms at octaves 256 through 8,192 were compared with pre-pressure audiograms taken under almost identical conditions. Although a few cases exhibit a disabling loss, the mean drop for the whole group is surprisingly slight. It should be borne in mind that none of the cases reported here had been subjected to pressure more than once. Acuity seems not to drop markedly with perforation of the drumskin, bleeding from the drumskin or external meatus, or separation of the layers of the drumskin. It seems to be associated less with otopathology as such than with the extent to which the bulla is filled with blood. Thus the sharp drops in acuity which do occur may perhaps, at least in their early stages, be referred in large part to damping of the ossicles.