Human T-cell lymphotropic virus type I (HTLV-I) causes adult T-cell leukemia and has recently been associated with HTLV-associated myelopathy/tropical spastic paraparesis. The HTLV-I is endemic throughout the Caribbean basin and parts of South America, and HTLV-associated myelopathy/tropical spastic paraparesis also seems to be common in this area. This 2-year study, 1985 and 1986, was designed to evaluate the occurrence of HTLV-I infection in all newly diagnosed cases of selected neurologic diseases in Panama City, Panama. Six (8%) of 71 patients had antibody to HTLV-I detected by immunofluorescence, enzyme-linked immunosorbent assay, radioimmunoassay, and Western blot assays; 5 patients' conditions were diagnosed as spastic paraparesis, and all 5 were seropositive and also had HTLV-I antibody in cerebrospinal fluid. The remaining seropositive patient had multiple sclerosis, and no antibody was detected in her cerebrospinal fluid. Clinical and electrophysiologic studies indicated that HTLV-I-associated spastic paraparesis is a multifocal, primarily demyelinating disease that principally involves the spinal cord.