Hyperglycemia-induced vascular inflammation resulting in the enhanced monocyte-endothelial cell (EC) interaction is the key event in the pathogenesis of atherosclerosis in diabetes. Here, we investigated the effect of isoflavone genistein on hyperglycemia-stimulated vascular inflammation. Human aortic EC (HAEC) were pretreated with genistein before the addition of high glucose (HG; 25 mmol/L) for 48 h. Genistein at a physiological concentration (0.1 μmol/L) significantly inhibited HG-induced adhesion of monocytes to HAEC and suppressed endothelial production of monocyte chemotactic protein-1 (MCP-1) and IL-8. Inhibition of adenylate cyclase or protein kinase A (PKA) significantly attenuated the antiadhesion effect of genistein. Consistently, genistein improved HG-impaired intracellular cAMP production and PKA activity in HAEC. Six-week-old diabetic db/db mice were untreated (db/db) or treated with a diet containing 1 g genistein/kg diet (db/db+G) for 8 wk. Their nondiabetic db/+ mice were used as normal controls. Circulating concentrations of MCP-1/JE and KC were significantly greater, whereas IL-10 concentrations were lower in db/db mice than those in normal mice. Dietary supplementation of genistein did not normalize but significantly suppressed the elevated serum concentrations of MCP-1/JE from 286 ± 30 ng/L to 181 ± 35 ng/L and KC from 321 ± 21 ng/L to 232 ± 20 ng/L while increasing that of IL-10 from 35 ± 4 ng/L to 346 ± 35 ng/L in db/db+G mice. Further, genistein treatment suppressed diabetes-induced adhesion of monocytes to EC by 87% and endothelial secretion of adhesion molecules. We conclude that genistein improves diabetes-caused vascular inflammation, which may be mediated through promoting the cAMP/PKA pathway.