The incidence of human T cell lymphotropic virus type 1 (HLTV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is not well defined in the literature. Several studies have reported different incidence rates, and recent publications suggest a higher incidence and prevalence of HAM/TSP. The interdisciplinary HTLV Research Group (GIPH) is a prospective open cohort study of individuals infected with HTLV-1/2. This study describes the demographic data and HAM/TSP incidence rate observed in 181 HTLV-1-seropositive individuals and compares the results with previous reports in the literature. HAM/TSP was diagnosed on the basis of the World Health Organization diagnostic criteria and De Castro-Costa et al. [Proposal for diagnostic criteria of tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). AIDS Res Hum Retroviruses 2006;22:931-935]. Seven HAM/TSP incident cases were observed during the follow-up. The HAM/TSP incidence density was 5.3 cases per 1,000 HTLV-1-seropositive cases per year (95% confidence interval: 2.6-10.9), with a mean follow-up of 7±4 years (range: 1 month to 15 years). HAM/TSP was more frequent in women in their 40s and 50s with probable infection via the sexual route. The HAM/TSP incidence density among HTLV-1-seropositive cases observed in the present study is higher than that in previous studies. HAM/TSP may be underdiagnosed in countries like Brazil where HTLV infection is prevalent. Orientation and prevent transmission of HTLV programs are needed. Currently, preventing HTLV-1 transmission is the most effective way to reduce the impact of HAM/TSP on society.