This study aimed to determine the seroprevalence of antibodies to AQP4 in patients with HTLV-1 associated myelopathy (HAM/TSP) and that of HTLV-1 antibodies in patients with neuromyelitis optica spectrum disorders (NMOSD) and to compare the clinical characteristics of a HAM/TSP and NMOSD in Brazilian patients. Here, AQP4 is linked to neuromyelitis optica.