We used immunocytochemical methods on sural nerve biopsies from 42 patients with peripheral neuropathy to identify mononuclear cells, determine whether lymphocytic infiltration occurs in a variety of neuropathies, and identify the subtypes of lymphocytes. Immunostained cells were present in 76% of nerve biopsies. CD3+ cells (T lymphocytes) were greatest in density (cells/mm2). In patients whose CD4:CD8 T cell ratio was measured also in blood and cerebrospinal fluid, the CD4:CD8 T cell ratio was similar in all three compartments. These findings suggest that T lymphocytes are frequently present in nerves obtained from patients with various types of neuropathies and raise questions about factors that attract T lymphocytes into nerve that may be important in pathogenesis.