Laminar shear stress activates NADPH oxidase in vascular endothelial cells (ECs), and the generated superoxide radicals (O<sub>2</sub><sup>–</sup>·) are known to be involved in intercellular adhesion molecule (ICAM)-1 expression. In this study, the role of a glycosphingolipid (GSL), lactosylceramide (LacCer), as a second messenger in the shear-induced O<sub>2</sub><sup>–</sup>· generation and ICAM-1 expression was examined. It is known that glucosylceramide synthase (GlcT-1) catalyzes the synthesis of glucosylceramide (GlcCer) from ceramide, and subsequently lactosylceramide synthase (GalT-2) synthesizes LacCer from GlcCer. We observed that exposing cultured human umbilical vein ECs (HUVECs) to fluid shear stress (20 dyn/cm<sup>2</sup> for 30 min) activated GalT-2. Shear stress also increased EC O<sub>2</sub><sup>–</sup>· generation, that peaked at 30 min, and surface ICAM-1 protein expression at 6 h post-shear. EC preincubation with the antioxidant N-acetylcysteine (NAC; 20 m M for 2 h) completely abolished the shear-induced O<sub>2</sub><sup>–</sup>· production and significantly inhibited ICAM-1 expression. EC preincubation with D-1-phenyl-2-decanoylamino-3-morpholino-1-propanol ( D-PDMP), an inhibitor of the GSL glycosyltransferases GlcT-1 and GalT-2, abrogated the shear-induced activation of GalT-2. D-PDMP also abolished the shear-induced O<sub>2</sub><sup>–</sup>· production and ICAM-1 expression. We conclude that laminar shear stress activates GalT-2 to produce LacCer. In turn, LacCer activates NADPH oxidase, which produces O<sub>2</sub><sup>–</sup>·, and O<sub>2</sub><sup>–</sup>· mediates the shear-induced increase in ICAM-1 expression. Thus, LacCer may play an important role in hemodynamic force-induced pathological conditions, such as atherosclerosis and ischemia/reperfusion injury.