Diabetic Lewis rats received pancreaticoduodenum allotransplants from Brown-Norway donors. Cyclosporine A (Cy-A) was used in a dose of 10 mg/kg/day for immunosuppression. These transplanted rats (n = 190) were compared with nondiabetic Lewis rats (n = 36), nondiabetic Lewis rats receiving 10 mg/kg/day Cy-A (n = 42), diabetic rats (n = 103), and diabetic rats receiving 10 mg/kg/day Cy-A (n = 45). The percentage area of periodic acid-Schiff (PAS) positive basement membrane (BM) of rectus muscle microvasculature was compared in each of the groups. It was found that the percentage area of PAS positive BM increased markedly over 15 months of uncontrolled diabetes. Cy-A did not have a significant effect on either normal or diabetic skeletal muscle vascular BM. Rats with established diabetes showed some reversal in the percentage area of PAS positive BM, when pancreas transplantation was performed at 9, 12, and 15 months of diabetes. Pancreas transplantation may appear beneficial even after the development of BM thickening of skeletal muscle vascular BM.