Introduction: Hyperbaric oxygen treatment (HBOT) involves some risk of central nervous system (CNS) oxygen toxicity, which may be revealed by various signs and symptoms including seizures in patients breathing O-2 at pressures of 2 ATA or higher. The aim of this study was to determine the incidence of such seizures in the Underwater and Hyperbaric Medicine Departments of two university hospitals. Methods: We retrospectively evaluated 80,679 patient-treatments for 9 clinical indications to determine the incidence of seizures attributable to CNS O-2 toxicity. Because different protocols were used for HBOT, the treatments were studied in four groups according to the chamber type used and the medical facility at which it was located. Results: Only 2 seizures were documented, yielding an incidence of 2.4 per 100,000 patient-treatments. Both cases occurred in a multiplace chamber pressurized to 2.4 ATA with O-2 delivered by mask for three x 30 min with 5-min air breaks. Discussion: The seizure incidence reported here is lower than other studies published in the literature. The delivery of O-2 by mask rather than hood may be a factor. Nevertheless, it appears that the risk of seizures due to CNS O-2 toxicity during HBOT is very low as long as appropriate exclusion criteria and treatment profiles are used.