Summary There is much evidence that diabetes and hyperglycaemia contribute to the impairment of endothelial function and induce severe changes in the proliferation, the adhesive and synthetic properties of endothelial cells. Induction of apoptosis could represent one mechanism to prevent the new accumulation of those vascular defects and to allow generation of vascular endothelium. In this study, we demonstrate that high concentrations of glucose or proinsulin induce apoptosis in human umbilical endothelial cells by three independent methods (DNA fragmentation, fluorescence activated cell sorting analysis, and morphology). The number of apoptotic cells was increased by glucose (30 mmol/l or proinsulin (100 nmol/l) from less than 10 % to about 30 %. Activation of protein kinase C (PKC) largely prevented the induction of apoptosis, whereas inhibition of PKC further increased the number of apoptotic cells. Similar changes as induced by glucose were also observed after incubation of the cells with the non-metabolisable 3-0-methylglucose. These findings indicate that hyperglycaemic conditions stimulate the induction of apoptosis in endothelial cells by a mechanism which is independent from the formation of diacylglycerol and the activation of PKC. The induction of apoptosis by the non-metabolisable glucose suggests that formation of oxygen derived radicals by autoxidative processes is involved and may lead to an activation of transcription factors such as nuclear transcription factor-kB (NF-kB) transferring the activation signal into the nucleus and leading to changes in gene expression necessary for induction of apoptosis. [Diabetologia (1998) 41: 249‐256]