In a patient with tropical spastic paraparesis associated with a positive titer for human T-lymphotropic virus type I, electrophysiological study detected a mixed, axonal and demyelinating, multifocal neuropathy. Perineural and perivascular infiltrates, moderate axon loss, wallerian degeneration, and demyelinating lesions of isolated fibers were present in the nerve biopsy specimen. These inflammatory lesions resembled those found in the central nervous system of patients with tropical spastic paraparesis.