Background The peritoneum begins to undergo morphologic changes before the start of peritoneal dialysis (PD), particularly in diabetic patients. The present study was conducted to investigate the effects of diabetes on the peritoneum. Methods This study involved 17 patients who began receiving PD and had diabetes as an underlying disease (DM group), and 30 patients without diabetes who served as a control group (nonDM group). At the start of PD, the parietal peritoneum was sampled to assess submesothelial connective tissue thickness, number of capillaries and postcapillary venules, and indications of vasculopathy (grades 0 – 3). Results Submesothelial connective tissue thickness was significantly greater in the DM group than in the nonDM group ( p < 0.01). The number of capillaries was significantly greater in the DM group ( p < 0.01). Based on multivariate linear regression analysis, diabetes was identified as a significant independent variable of both submesothelial connective tissue thickness and number of capillaries ( p < 0.01). Conclusions In diabetic patients, morphologic changes of the peritoneum are marked at the start of PD.