We undertook histological and immunopathological studies on biopsied sural nerves from 9 patients with chronic inflammatory demyelinating polyneuropathy (CIDP). The diagnosis of CIDP was based on the research criteria proposed by the Ad Hoc Subcommittee of the American Academy of Neurology AIDS Task Force. The nerve pathology in these patients comprised macrophage-associated active demyelination and subsequent remyelination of various proportions. The presence of T cells in the endoneurium correlated with activity of demyelination. An analysis of T cell subsets demonstrated that the number of CD8-positive cells predominated over that of CD4-positive ones. Infiltration of B cells, and depositions of immunoglobulin and complement were not seen. These observations suggest that a T cell-mediated process is of pathogenic significance in CIDP. Furthermore, a double immunofluorescence staining revealed that most HLA-DR antigen-positive cells in the nerves in which active demyelination was seen coexpressed a macrophage-specific determinant. Conversely, HLA-DR-positive Schwann cells were found in the nerves in which remyelination was predominant. The expression of HLA-DR antigen on Schwann cells might not play a pathogenic role in the active demyelination in CIDP.