We examined frozen sections of frontal cortex, medulla, and dorsal root ganglia from a patient with small-cell lung cancer and paraneoplastic encephalomyelitis, involving the medulla and dorsal root ganglia, with a panel of antibodies reactive for IgG, IgM, C3, B cells, T cells, T cell subsets, macrophages, and class I and II (HLA-DR) major histocompatibility complex (MHC) antigens. We detected an antineuronal antibody (anti-Hu) in the serum and CSF of the patient and found deposits of IgG in the periphery of some neurons in dorsal root ganglia. The infiltrates were almost exclusively T cells with a predominance of CD8-positive cells. Neurons did not express class I or II MHC antigens. Satellite cells in the dorsal root ganglia from the patient and controls were HLA-DR-positive. These data indicate that CD8-positive T cells predominate in the inflammatory infiltrates of paraneoplastic encephalomyelitis. IgG deposits may be relevant in the damage of the sensory neurons.