The human T lymphotropic virus, type-I is associated with a slowly progressive neurologic disorder termed human T lymphotropic virus, type-I associated myelopathy/tropical spastic paraparesis. Considerable information on the pathology, epidemiology, virology, and immunology is known about this disorder and will serve as a framework to define the role that a human retrovirus can play in human disease. In particular, this review will discuss the role of cellular immune responses to the virus in patients with human T lymphotropic virus, type-I associated myelopathy/tropical spastic paraparesis and how these responses may be associated with the pathogenesis of this disorder. By defining virus-specific functional cellular host responses we hope to improve our understanding of the underlying mechanisms that may be involved in the neuropathology of human T lymphotropic virus, type-I associated neurologic disease.