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Background. Human T-cell lymphotropic virus type 1 (HTLV-1) is the agent of HTLV-1–associated myelopathy/tropical spastic paraparesis (HAM/TSP), observed in up to 5% of infected individuals. Despite low prevalence, many HTLV-1–infected patients who do not fulfill criteria for HAM/TSP present with neurological complaints related to sensory, motor, urinary, or autonomic manifestations. The aim of this study was to determine the incidence of neurologic manifestations and risk factors associated with these outcomes.Methods. The incidence of HAM/TSP and new signs and neurologic symptoms were computed in a group of patients enrolled in a cohort study.Results. Of 414 subjects, 76 had definite HAM/TSP, 87 had possible or probable HAM/TSP, and 251 subjects had no neurologic manifestation and were selected for analysis. Definite HAM/TSP developed in 5 (1.47%) patients. Follow-up of at least 3 years was achieved in 51% of patients. The incidence rate was computed in 1000 person-years (206 for hand numbness, 187 for feet numbness, 130 for nocturia, and 127 for urgency). Average incidence rate in neurological exam was 76 for leg hyperreflexia, 53 for leg weakness, and 37 for Babinski sign. In the applied Expanded Disability Status Scale, the incidence rate of worsening 1 point was 134 per 1000 person-years. Kaplan–Meier curves stratified by sex and proviral load showed that females and patients with proviral load >50 000 copies/106 peripheral blood mononuclear cells had a higher risk of progression.Conclusions. Development of neurological symptoms or signs occurred in up to 30% of asymptomatic subjects during 8 years of follow-up.