Perineurial cell basement membrane (PCBM) thickening is a consistent feature in diabetes mellitus (DM) and may have relevance to the cause of DM neuropathy. In this ultrastructural morphometric study of identical twins discordant for DM, we found that the PCBM was significantly thicker in the dermal nerves of the diabetic twin. Following pancreas transplantation (PT) and a 2-year period of euglycemia, the PCBM in both dermal and sural nerves was significantly thinner. At the end of the 2nd year post-PT, the PCBM thickness in the dermal nerves of the diabetic was not significantly different from the non-DM twin. The correction of diabetic dysmetabolism may have played a role in the regression of PCBM. These data suggest that PCBM thickening may not be a permanent legacy of DM.