HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is apparently a disease with a chronic evolution without spontaneous remissions. The real profile of its natural history and of the progression of the neurological disability, however, awaits confirmation. We devised the present study to evaluate the progression profile of the neurological disability of HAM/TSP in a series of 43 patients who have never received any kind of previous immune therapy. Patients were divided into different groups according to the duration of their disease. Age, gender and the Kurtzke's disability status scale (DSS) at the time of the first examination were compared. There were no statistically significant differences among groups with different disease duration. The present study suggests that the evolution of the neurological disability in HAM/TSP occurs mainly during the first year of the disease and becomes relatively stable after that. Therefore we speculate that the variable therapeutic success rates observed in many series of the literature could be due to the timing in the beginning of the pharmacological immunosuppression. Probably the therapeutic window in HAM/TSP lies within the first year of the disease. Thus it might be of utmost importance that future therapeutical trials take into consideration the duration of the disease since this factor can play an important role in the results of the trial.