We conducted a prospective study to determine the incidence of middle ear barotrauma in patients who were undergoing hyperbaric oxygen therapy (HBOT). We also investigated the value of the nine-step inflation/deflation test and otoscopic findings before and immediately after the initial HBOT session in predicting barotrauma in an attempt to establish some criteria for prophylaxis. The study was conducted on 36 ears of 18 adults who had no history of eustachian tube dysfunction. Patients were being treated with HBOT for sudden hearing loss, wound-healing complications, or complications of diabetes. After 7 days of HBOT, barotrauma was seen in 12 of the 18 patients (66.7%) and in 18 of the 36 ears (50.0%). The nine-step inflation/deflation tests, which were performed before and immediately after the initial HBOT session, were not predictive of barotrauma (p = 0.095 before and p = 0.099 after). However, otoscopic findings obtained immediately after the first session of HBOT were predictive of barotrauma, with a sensitivity and specificity of 83 and 100%, respectively. We conclude that patients with even minor positive pathologic findings on otoscopy immediately following HBOT are at increased risk of middle ear barotrauma if HBOT is to be continued without prophylaxis.