To clarify the involvement of 5-hydroxytryptamine (5-HT) in promotion of thrombogenesis in diabetes, we examined the inhibitory effect of sarpogrelate, a 5-HT(2A) receptor antagonist, on thrombus formation in diabetic rats. In streptozotocin-induced diabetic rats, polyethylene tube-induced thrombus formation was enhanced compared with that in normal rats. The thrombogenesis was inhibited by sarpogrelate; cilostazol, a PDE3 inhibitor; and aspirin, a COX inhibitor, by 75.8%, 42.3%, and 34.3%, respectively. The inhibition by sarpogrelate was more pronounced in diabetic rats than normal ones. High glucose and 5-HT increased the expression of vascular cell adhesion molecule-1 (VCAM-1) in human umbilical vein endothelial cells (HUVECs) and combination of both high glucose and 5-HT further potentiated the effect. Sarpogrelate but not aspirin inhibited the increase in VCAM-1 expression induced by high glucose and 5-HT. These findings suggest that 5-HT mediates the enhanced thrombogenesis in diabetes and suggests that a 5-HT(2A) receptor antagonist may have novel therapeutic potential for the treatment of diabetic complications.