We immunologically examined the pathogenesis of chronic inflammatory demyelinating polyradiculoneuropathy in a patient with HB hepatitis. A 41-year-old male clerk has been suffered from muscle weakness, tingling and numbness in the distal portion of all limbs. All symptoms were compatible with the typical patterns of chronic inflammatory demyelinating polyradiculoneuropathy. We examined the patient's serum and biopsied sural nerve, using histochemical and immunological techniques. We detected the band that reacted with anti-HBs antibody in the sural nerve in western blotting. The result indicated that HBs antigen was expressed on the peripheral nerves in the patient. There were no anti-peripheral nerve antibodies neither in the sural nerve or serum. There was no increase of immune complex in the serum. No deposition of immunoglobulins and complements were detected in the sural nerve. Immunoadsorption therapy had no effect on this patient, but administration of prednisolone improved his symptoms drastically. These findings suggest that a cytotoxic T cell may had played a more important role than humoral factors in this patient's nerve injury. Though the pathogenesis of chronic inflammatory demyelinating polyradiculoneuropathy remains unclear, our findings seem to be very interesting in that they go some way toward clarifying the pathogenesis of this disease.