Lumbar dorsal root ganglia were sampled at autopsy from 27 diabetics and 22 nondiabetics. Using quantitative ultrastructural morphometry, the perineurial cell basement membrane thickness was estimated for each case, and a highly significant difference was found between the patient means for each group (nondiabetics 114 +/- 18 nm vs. diabetics 203 +/- 64 nm, P less than 0.001). These results indicate that perineurial cell basement membrane thickening probably is present diffusely throughout the diabetic peripheral nervous system because it has been reported in the sural nerves and dermal nerves. The increased permeability of the blood-nerve barrier of the dorsal root ganglia is apparently not a factor in the development of this basement membrane hypertrophy.