We studied the impact of diabetes mellitus (DM) on the radial artery (RA) in 30 patients with DM and 30 non-diabetic patients undergoing coronary artery bypass grafting with autologous RA. RAs were recorded as normal if there was no cellular or stromal tissue between the endothelium and the internal elastic lamina. The RA was normal in 26.7% of diabetic and 76.7% of non-diabetic patients ( p = 0.000298). Intimal thickness index and intima:media ratio were higher in the former than in the latter ( p < 0.05; p < 0.05), with no significant difference in luminal narrowing ( p > 0.05). Electron microscopy scores were lower in the non-diabetic group ( p < 0.001); endothelial nitric oxide synthase (eNOS) protein expression and optical density were higher ( p < 0.001). Von Willebrand factor and endothelin-1 messenger RNA (mRNA) levels were higher in the DM patients ( p < 0.001). The quality of the RA in patients with DM was thus inferior to that in non-diabetic patients. Care should be taken when selecting RA as a conduit in patients with DM.