Intercellular adhesion molecule 1(ICAM-1) expression and cellular changes in human umbilical vein endothelial cells(HUVEC) and ECV304, an established cultured line derived from HUVEC, under hypoxia and hypoxia(H)/reoxygenation(R) were investigated by immunological, cytochemical, and morphological methods. ICAM-1 expression in HUVEC decreased slightly under hypoxia(92%) and was up-regulated under reoxygenation(114%), but this up-regulation was diminished by superoxide dismutase(SOD). The up-regulation of ICAM-1 expression by interleukin-1 beta(IL-1 beta) was detected at almost equal levels under hypoxia, and H/R. Using lucigenin-chemiluminescence, we demonstrated superoxide generation from HUVEC under H/R. The Ca2+ influx under hypoxia, and the Ca2+ release from the cells under H/R reduced by SOD were detected cytochemically. Vacuole formation as cell injury under hypoxia and H/R was detected by electron microscopy. The present findings provide evidence that superoxide generated from HUVEC is responsible for the up-regulation of ICAM-1 expression under H/R, and the cause of endothelial cellular injury.