Hyperbaric oxygen (HBO) is 100% oxygen administered at elevated atmospheric pressure to patients with inflammatory diseases. We developed an in vitro model to investigate the effects of HBO on stimulus-induced proinflammatory cytokine transcription and translation. Human blood-derived monocyte-macrophages were stimulated before being transferred to an HBO chamber where they were incubated at 97·9% O2, 2·1% CO2, 2·4 atmospheres absolute, 37°C. Controls were maintained in the same warm room at normoxia at sea level, hyperoxia or increased pressure alone. A 90-min HBO exposure inhibited IL-1β synthesized in response to lipopolysaccharide by 23%, lipid A by 45%, phytohaemagglutinin A (PHA) by 68%, and tumour necrosis factor (TNF)-α by 27%. HBO suppressed lipopolysaccharide-, lipid A- and PHA-induced TNF-α by 29%, 31% and 62%, respectively. HBO transiently reduced PHA-induced steady state IL-1β mRNA levels. Hyperoxia alone and pressure alone did not affect cytokine production. The immunosuppressive effect of HBO was no longer evident in monocyte-macrophages exposed to HBO for more than 3 h. Interestingly, cells exposed to HBO for 12 h synthesized more IL-1β than cells cultured under control conditions. In summary, HBO exposure transiently suppresses stimulus-induced proinflammatory cytokine production and steady state RNA levels.