Structural abnormalities of the perineurium from six spontaneously diabetic dogs (diabetes duration 4–8 years and six control animals were quantified using detailed electron microscopic morphometric methods on superficial peroneal nerve biopsy specimens. Total perineurial sheath thickness (μm) was significantly increased in diabetic (8.8 ± 0.6) compared to control animals (6.2 ± 0.3) (P < 0.02). This was attributed to a significant increase in the mean perineurial lamellar width in diabetic (0.49 ± 0.03) compared to control (0.40 ± 0.01) (P < 0.04) animals. The number of lamellae also showed a non-significant increase in diabetic animals (7.8 ± 0.4) compared to controls (6.9 ± 0.13) (P < 0.06). There was no change in the mean interlamellar space in diabetic (0.7 ± 0.05) compared to control (0.6 ± 0.06) (P = 0.15) animals. The total interlamellar space was increased in diabetic (5.7 ± 0.5) compared to control (4.1 ± 0.36) (P < 0.04) animals. The perineurial cell basement membrane thickness (nm) was significantly increased in diabetic (126.9 ± 9.8) compared to control (62.8 ± 6.1) (P < 0.005) animals. The current study has demonstrated significant abnormalities of the perineurium in the spontaneously diabetic dog, which may have relevance to the pathogenesis of diabetic neuropathy.