This study examined the effects of hyperglycemic/hyperosmolar conditions on neutrophil adhesion to endothelium. A 2.4-fold increase in neutrophil adhesion was observed following exposure of endothelial cells to 100 mM glucose for 24 hours (n = 9, p less than 0.001), and a 57% increase occurred following pre-incubation in 50 mM glucose for 24 hours (n = 8, p less than 0.01). Phase-contrast microscopy of endothelial monolayers following exposure to 100 mM glucose showed no gross morphological changes, and no differences in membrane permeability were detected. Electron microscopy, however, revealed that cells exposed to 100 mM glucose generally had more pseudopods, an increased density of cytoplasm, and more finely dispersed chromatin throughout the nucleus. Increased neutrophil adhesion to endothelium was also observed following exposure of endothelial cells to 100 mM mannitol, indicating that hyperosmolarity could account for the effect observed with high concentrations of glucose. However, the increase following mannitol treatment was 170% of control (n = 6), while glucose pre-treatment resulted in a 240% increase (n = 9, p = 0.01 for glucose compared to mannitol). The effect of high concentrations of glucose on neutrophil adhesion to endothelial cells was to some degree specific for endothelium, as the same effect was not observed with microvascular pericytes. As neutrophil adhesion to endothelium is an important early step in the acute inflammatory response, these findings may help to explain the increased incidence and severity of infections in diabetic patients.