Study design: Cross-sectional study. Objectives: To determine the prevalence of falls in human T-cell lymphotropic virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients and possible factors associated to their occurrence. Setting: Instituto de Pesquisa Clı´nica Evandro Chagas, Fundac¸a˜o Oswaldo Cruz (FIOCRUZ) - Brazil. Methods: Thirty-six HAM/TSP patients able to walk at least 20 m were assessed by a questionnaire. Data regarding gender, age, duration of disease (DD), HTLV-I proviral load (HPL), frequency of physical activity (FCA), use of walking aids, functional ambulation level, the number of falls and associated injuries in the last year were reviewed. Multiple correspondence analysis was used to group characteristics of this sample according to the fall occurrence. Results: The prevalence of falls was 63.9% and we observed injuries in 47.8% of the cases. Four groups were identified in the descriptive analysis. One group was formed by faller individuals, men < 60 years, independent ambulation, FCA ≥ 3 times per week and HPL < 6.6 copies per 100 cells (group B). The other one comprised non-faller patients, women < 60 years, restricted ambulation, DD ≥ 7 years, use of orthosis, FCA 0-1 time per week and HPL ≥ 6.6 copies per 100 cells (group D). The others two groups comprised individuals that did not use orthosis (group A) and those that FCA was two times per week and DD < 7 years (group C). Conclusion: Falls occur in roughly two-thirds of ambulatory HAM/TSP patients and are associated with significant morbidity. Further studies with a larger number of patients are necessarily to identify risk factors in order to elaborate specific programs to prevent falls in this population.