Morphometric studies of biopsied sural nerves were performed among 16 diabetic patients (14 with polyneuropathy and 2 without polyneuropathy). Main clinical findings at the time of biopsy were reviewed retrospectively and the correlations between clinical findings (the presence or absence of sensory disturbance, motor disturbance, retinopathy and pain, and the duration of sensory disturbance) and nerve fiber densities were analyzed. The following results and conclusions were obtained. 1) Decrease of densities of both myelinated and unmyelinated fibers was found in 9 out of 16 cases. Decrease of the density of unmyelinated fibers in diabetic patients was noticeable among over 100 cases of various neuropathies studied by us. Both myelinated and unmyelinated fibers seem to be decreased with the duration of sensory disturbance. 2) In teased fiber analysis myelinated fibers showing axonal degeneration as well as segmental demyelination were seen at relatively lower frequency. Onion bulb formation was found only in one case out of 16 cases. Therefore it was concluded that axonal degeneration is probably the main pathological change in sural nerves in this study, considering the decrease of nerve fiber densities as well. 3) The mean thickness of the perivascular space of endoneurial small vessels in diabetic patients was greater than that of the control (32 consecutive patients of various neuropathies). 4) No good correlations were found between the presence or absence of motor disturbance, pain and retinopathy, and the nerve fiber densities.