The prolonged effects of glucose and insulin on cultured human microvascular endothelial cells from omental tissue (HOMEC) were observed to identify the contribution of sustained hyperglycemia and/or hyperinsulinemia to the pathogenesis of diabetic microangiopathy. When the cells were cultured for 10 days in Medium 199 with 100 or 500 mg/dl glucose, the number of cells was reduced to 78% in the culture of 500 mg/dl glucose as opposed to that of 100 mg/dl glucose. The difference in the number of cells between these two groups became obvious between the 5th and the 7th culture days. The replacement of D-glucose with L-glucose did not show any reduction in the number of cells, indicating the impertinence of high osmolarity, induced by high glucose (305 mOsm/kg) to the number of cells. This reduction resulted from the cellular damage during the culture period rather than the retardation of growth, according to the experiments of [3H]thymidine uptake and the 51Cr release assay. Since the uptake of glucose, measured as the uptake of 3-O-methyl-alpha-D-glucose, was higher and the Na+/K+ pump activity decreased in high glucose condition, it is suggested that the excessive intracellular accumulation of glucose caused the damage of cells through the disturbance of ion exchange. Insulin augmented the reduction in the number of cells induced by high glucose when supplemented together for 10 days at concentrations of 10(-6)-10(-12)M. The uptake of glucose increased further to 154% by the addition of insulin to high glucose as compared to that of high glucose alone, however, the decreased Na+/K+ pump activity by high glucose was restored to the control level by insulin. The aggravating effect of insulin to the cellular damage induced by high glucose seems to be mediation via the mechanism other than the decreased Na+/K+ pump activity. In conclusion, HOMEC were gradually damaged by high glucose and by insulin, and hyperglycemia and hyperinsulinemia would be of pathogenetic importance for diabetic microangiopathy.