Atherosclerosis is enhanced in humans with diabetes mellitus, but the mechanism(s) involved remains unclear. Increased leukocyte-endothelium interaction may be involved, since mononuclear leukocyte adherence to the endothelium is an early event in both experimental atherosclerosis and alloxan-induced diabetes in rabbits. In situ immunohistochemistry was used in en face Häutchen endothelial preparations to identify endothelial cells that stained with antibodies to endothelial leukocyte adhesion molecules (vascular cell adhesion molecule-1 [VCAM-1] and E-selectin), and the number of stained cells per 10,000 cells was determined. Preparations from aortas of diabetic normolipemic and egg yolk diet-induced hyperlipemic diabetic rabbits were compared with those from normoglycemic animals on similar diets. Cross sections of the vessel wall were stained with oil red O and antibodies to VCAM-1, E-selectin, and RAM-11-positive macrophages. After 4 weeks of hyperlipemia the frequency of cells expressing VCAM-1 or E-selectin was significantly increased compared with normolipemic controls; this frequency was further increased in the aortas of hyperlipemic diabetic rabbits. VCAM-1 and E-selectin expression was more frequent in normolipemic diabetic rabbit aortas than in hyperlipemic, normoglycemic vessels. The potentiation of expression of these adhesion molecules in diabetic animals may provide part of the explanation for the enhanced atherosclerosis associated with diabetes mellitus.