The frequency and nature of complications encountered during hyperbaric oxygenation (HBO) therapy have been studied in 678 patients with different diseases who were subjected to 4219 HBO procedures. The complications were observed in 17.9% of patients and were divided into three groups. Group I, complications associated with the damaged function of Eustachian tube and middle ear (7.2%); group II, complications associated with recurrence of chronic diseases and postoperative condition (8.1%); group III, complications associated with the impact of elevated O2 pressure (2.6%). The majority of complications (15.3%) were functional in nature. To prevent the onset of complications it seems necessary to perform differentiated therapy prior to HBO, and in some patients during sessions and between them.