The objective of this study was to evaluate exhaled nitric oxide concentration (FENO) and exhaled breath condensate (EBC) pH and H2O2 as biochemical markers of pulmonary oxygen toxicity in association with hyperbaric oxygen (HBO2) therapy. FENO, EBC pH and H2O2 were measured during the course of a 4 week HBO, treatment period, and the responses to a single HBO2 exposure at the start and end of the treatment period were assessed. The HBO2 exposure was at a pressure of 240 kPa for 90 min 5 days a week for 4 weeks. Eight patients undergoing HBO2 therapy and eight control subjects participated in the study. There was a reduction in FENO immediately after HBO2 exposure of 33.1 (SD = 7.8) % on Day 1 and 40.7 (SD = 8.9) % on Day 25. EBC pH was reduced after the first exposure only. Baseline F(E)NO and EBC pH and H2O2 measured before the HBO2 exposures did not change throughout the HBO2 treatment period. A single HBO2 exposure induces a significant transient decrease in FENO. Repeated exposures do not appear to induce inflammatory processes in the lung associated with an increase in FENO.