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.