The authors has treated 5 cases of sudden deafness in 1977, using hyperbaric oxygen therapy alone with 2 atmospheric pressure instead of combining with vasodilator, vitamins, and steroid, which were usually used in the past. Hyperbaric oxygen therapy had been used one man chamber, keeping up positive acceleration pressure in 0.1kg/cm/min. raised maximum to 2 atmospheric pressure of oxygen, and negative acceleration pressure in 0.1kg/cm/min.. The duration from the acceleration to reduction was taken one hour and treatment was done one time per day for 15 to 20 days. All the cases showed rapid and good effect within one week in duration 3 of 5 cases were found hearing gain of 30 dB on the speech range and the other one case was remained 35 dB and another one was 45 dB. Side effect of all cases were otalgia and tinnitus during hyeperbaric oxygen therapy, but disappeared after treatment in 3 to 5 times and other side effect was not experienced.