Objective: Human T-lymphotropic virus type-I-associated myelopathy / tropical spastic paraparesis (HAM/TSP) is a rare neurologic disorder that affects less than 5% of patients infected with the HTLV-I virus. The purpose of this protocol is to study the natural history of HAM/TSP by monitoring clinical progression of patients longitudinally. Additionally, we will attempt to define the virological and immunological changes of HAM/TSP. Study Population: Patients with HAM/TSP who fulfill World Health Organization diagnostic criteria are eligible to participate in this protocol. Asymptomatic seropositive individuals and individuals with indeterminate HTLV-1 serology are also eligible to participate. Design and Outcome Measures: A longitudinal assessment of clinical, virological and immunological progression in HAM/TSP will be accomplished through periodic testing and evaluation. Asymptomatic seropositive individuals, those with seroindeterminate HTLV-I serology and normal volunteers may serve as controls. Longitudinal standardized neurological examinations will be performed. Longitudinal samples of serum, plasma, and lymphocytes may be obtained from participants. Lumbar punctures may be performed on all participants. These samples will be used virological and immunological assays. A focus is on the relationships between the characteristics of viral infection, the immune response, and the genetic makeup.