Ultrastructural and teased-fiber studies were carried out on the sural nerves of nine patients with beriberi neuropathy Axonal degeneration was the most prominent feature, and large myelinated fibers were more affected than unmyelinated ones. An unusual change with accumulation of flattened sacs or tubuli was recognized in the axoplasm of myelinated fibers of untreated patients. Active regeneration was extensive in patients receiving vitamin B. Segmental demyelination, remyelination, and early onion bulbs were scarece in patients with a long and relapsing course before treatment.