We have examined the peripheral nerves in four patients with polyarteritis nodosa. They were consisted of two males and two females with the age of 70, 49, 65 and 75 years respectively. The sciatic and posterior-tibial nerves, sural nerves, ventral and dorsal spinal nerve roots, dorsal ganglia and spinal cord were removed at autopsy. Particularly, the sciatic and posterior-tibial nerves were removed as a whole and suspended in a glass cylinder with a weight and fixed in 1.5% glutaraldehyde and 0.05 M phosphate buffer, pH 7.4 for 16 hours. The nerve fragments of every 3 to 3.5 cm along the sciatic and posterior-tibial nerve were examined on conventional paraffin-embedded and epon-embedded sections, and osmicated teased-fibers. Necrotizing angitis was commonly present in the epineurium of the sciatic and posterior-tibial nerves in all four patients. Although necrotizing angitis was diffusely distributed in the proximal to distal portions of the nerve, loss of myelinated fibers, occurred only in the distal to mid-lower portions of the sciatic nerve. There was no substantial myelinated fiber loss in the proximal part of the sciatic nerve in three of four cases. Myelinated fiber loss in the fascicle was central fascicular or multiple-focal in pattern in the proximal portions of the nerve, but was diffuse in more distal portion of the posterior tibial nerve. Segmental demyelination and myelin irregularity in the teased-fiber preparation were more prominently observed in the proximal portions, but fibers with axonal degeneration were more frequent in the distal portions of the nerve.(ABSTRACT TRUNCATED AT 250 WORDS)