Thin-walled channels resembling lymphatic capillaries and containing lymphocytes were observed in perivascular spaces throughout the central nervous system of patients with various neurological disorders. This suggests that immunological surveillance in the central nervous system may normally involve lymphocyte circulation through the perivascular compartment. In some old multiple sclerosis plaques, perivascular lymphoid tissue was found which was organized in a manner similar to the antibody-producing medullary region of lymph nodes. This may indicate continuous processing of the putative multiple sclerosis antigen in such lesions.