Diabetes is associated with abnormal angiogenesis. Increased angiogenesis contributes to severe forms of diabetic retinopathy, but angiogenesis is decreased in response to myocardial ischemia in diabetic patients. We evaluated the direct effect of hyperglycemia on angiogenesis in the chicken chorioallantoic membrane assay, a model of active neoangiogenesis. Hyperglycemia, lasting up to 7 days, was induced in 7-day-old chick embryos by a single intravitellus glucose injection. Control embryos received either water (volumic control) or mannitol (osmotic control). Hyperglycemia decreased angiogenesis in this model from the 5th day on. The pattern and expression level of the main vascular growth factors' genes were not altered by hyperglycemia, as assessed by in situ hybridization and semiquantitative RT-PCR. As early as 2 days after hyperglycemia was induced, an increased apoptosis of endothelial cells and pericytes was detected by transferase-mediated deoxyuridine triphosphate nick-end labeling assay and electron microscopy. In the meantime, endothelial cell proliferation was decreased, as assessed by incorporation of bromo-deoxyuridine. Hyperglycemia can therefore impair angiogenesis without altering the expression level of vascular growth factors through induction of apoptosis and decreased proliferation of endothelial cells.