BACKGROUND: Although hyperbaric oxygen (HBO) -100% oxygen at two to three times the atmospheric pressure at sea level-has a number of beneficial biochemical, cellular, and physiologic effects, it is intrinsically associated with the potential for producing mild to severe toxic effects. The contribution of the cellular adhesion molecules and macrophages in the renal oxygen toxicity is not well understood. Thus, we have investigated the toxic effect of HBO expressed by the analysis of intercellular adhesion molecule-1 (ICAM-1) and infiltration of macrophages in rat kidney. METHODS: Male Sprague-Dawley rats weighing about 250 g were exposed to HBO at 3 ATA of 100% O2 for 4 hours. The expression of ICAM-1 and infiltration of CD68-positive macrophages were serially observed by immunohistochemical analysis. RESULTS: At 3 days after HBO exposure, CD68-positive macrophage counts were increased in glomeruli and tubulointerstitium of kidney. The expression of ICAM-1 was enhanced 1 day after HBO exposure and increased more for 3 days. There was a significant correlation between ICAM-1 expression and macrophage accumulation in the glomeruli. At 7 days after HBO, those alterations recovered to normal status. CONCLUSION: The 4-hour HBO exposure induced ICAM-1 expression and macrophage accumulation in the kidney and these change lasted for 3 days. Therefore, sustained activation of macrophages in renal oxygen toxicity may occur after prolonged (more than 4 hours) or repetitive exposures to HBO.