The frequency and importance of dysautonomia in human T-cell lymphotrophic virus type I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) have not been fully investigated. We describe the characteristics of dysautonomia in such patients in a case-control study. Our results indicate that autonomic disturbances are more frequent in HAM/TSP than has been previously suggested, with a predominance of sympathetic nervous system dysfunction. In some of these patients, the symptoms may be severe enough to warrant specific treatment.