The purpose of this study was to evaluate the effects of insulin on leukocyte-endothelial cell adhesion in the retinal microcirculation in vitro and in vivo. Human retinal endothelial cells (HRECs) were cultured in medium with or without insulin, and neutrophils allowed to adhere. Adherent neutrophils were quantified by measuring myeloperoxidase activity. Surface expression of endothelial adhesion molecules were studied with the use of an enzyme immunoassay. Insulin at concentrations of 50 and 100 microU/ml significantly increased neutrophil adhesion to HRECs compared with the control cells (P < 0.01, respectively). Surface expression of intercellular adhesion molecule-1 (ICAM-1) significantly increased when HRECs were exposed to 100 microU/ml insulin, as compared with the control cells (P < 0.05). Anti-ICAM-1 antibody significantly inhibited neutrophils adhesion to HRECs (P < 0.0001). Brown-Norway rats received subcutaneous injection of 0.2 U per 100 g body weight insulin three times. Control rats received the same amount of phosphate-buffered saline. Leukocyte entrapment in the retina was evaluated using acridine orange leukocyte fluorography. The number of leukocytes trapped in the retina of insulin-treated rats was significantly elevated compared with that in the control animals (P < 0.0001). These results suggested that insulin enhances leukostasis in retinal microcirculation. Hyperinsulinemia may be a risk factor of retinal microcirculatory disturbances.