Objective: The aim of this study is to evaluate the efficiency of Hyperbaric oxygen therapy (HBOT) in idiopathic sudden sensorineural hearing loss (ISSHL) and to define the factors related to the ISSHL that most improve with HBOT such as: onset of HBOT, hearing severity, audiometric shape and related symptoms (vertigo and tinnitus). Methods: Retrospective review of the clinical data of 394 patients treated with HBOT with the diagnosis of ISSHL in one single hyperbaric unit from 2005 to 2009. Audiograms from before and after HBOT were examined. Age, time onset of disease, hearing loss severity, audiometric shape, and related symptoms were reviewed. Recovery according to Siegel criteria was noted. Results: In 291 patients (73,8%) HBOT was started within 8 weeks of onset, while 87 (22%) HBOT was started after 8 weeks of ISSNL. 107 patients (27%) had cophosis and 190 (48%) had moderated hearing loss. The most frequent audiometric shape was the type E with 118 patients, followed by 79 patients with Type B. There was hearing recovery in 280 (71%) patients. According to Siegel’s classification, 113 (29%) patients had total recovery, 74 (19%) had partial recovery, 77 (19%) light recovery and 130 (33%) had no recovery Conclusio