Aims/hypothesis. Diabetes accelerates cardiovascular disease caused by atherosclerosis. Accordingly, diabetes accelerates atherosclerotic lesion progression and increases arterial smooth muscle cell proliferation. We hypothesized that diabetes can exert growth-promoting effects on smooth muscle cells via increased advanced glycation end-products or by dyslipidaemia. Methods. Primary human arterial smooth muscle cells were stimulated with advanced glycation end-products, other ligands of the receptor for advanced glycation end-products or fatty acids common in triglycerides. Cell proliferation was measured as DNA synthesis, cell cycle distribution and cell number. Effects of oleate on cellular phospholipids, diacylglycerol, triglycerides and cholesterol esters were analyzed by thin-layer chromatography, and oleate accumulation into diacylglycerol was confirmed by gas chromatography. Results. Human arterial smooth muscle cells express the receptor for advanced glycation end-products, but its ligands N ε -(carboxymethyl)lysine-modified proteins, methylglyoxal-modified proteins, S100B polypeptide and amyloid-β (1-40) peptide, exert no mitogenic action. Instead, oleate, one of the most common fatty acids in triglycerides, enhances platelet-derived growth factor-BB-mediated proliferation and oleate-containing 1,2-diacylglycerol formation in smooth muscle cells. This mitogenic effect of oleate depends on phospholipase D activity and is associated with an increased formation of oleate-enriched 1,2-diacylglycerol. Conclusion/interpretation. Oleate, not ligands of the receptor for advanced glycation end-products, acts as an enhancer of human smooth muscle cell proliferation. Thus, lipid abnormalities, rather than hyperglycaemia, could be a major factor promoting proliferation of smooth muscle cells in atherosclerotic lesions. [Diabetologia (2003[Diabetologia ( ) 46:1676[Diabetologia ( -1687 Keywords Atherosclerosis, diabetes, diacylglycerol, fatty acids, platelet-derived growth factor, phospholipase D, triglycerides. Atherosclerosis is accelerated by both Type I and Type 2 diabetes [1] by mechanisms that are poorly understood. Proliferation of arterial smooth muscle cells (SMCs) in lesions of atherosclerosis plays an important role in progression of early fatty streaks to fibroatheromas. We have recently reported that increased SMC proliferation in fibroatheromas occurs concomitantly with hyperglycaemia and an increased plasma triglyceride concentration in a porcine model of diabetes-accelerated atherosclerosis [2]. However, increased plasma glucose concentrations do not directly stimulate proliferation of SMCs isolated from these animals or from human subjects [2]. In this study, we investigated advanced glycation end-products (AGEs)