Human T-cell lymphotropic virus type 1 (HTLV-1) is a causative agent of adult T-cell leukemia and HTLV-1–associated myelopathy/tropical spastic paraparesis. HTLV-1–associated myelopathy/tropical spastic paraparesis is a chronic inflammatory disease characterized by loss of motor movement in response to spinal marrow cell destruction by T lymphocytes. To perform their cellular function, T cells need to be activated by antigenpresenting cells, such as dendritic cells (DCs). The aim of this work was to analyze DC differentiation and activation from monocytes of HTLV-1–infected individuals. We demonstrated that monocytes from HTLV-1–infected patients who had been stimulated to differentiate had an impaired loss of CD14 expression, expressed low levels of CD1a, and maintained secretion of tumor necrosis factor- compared with monocytes from noninfected donors. We further evaluated DC activation by tumor necrosis factor- α. We observed that in response to activation, DCs that were derived from noninfected donors had an increase in the percentage of CD83, CD86, and human leukocyte antigen-DR cells, whereas in DCs derived from HTLV-1–infected patients, the percentage of CD83, CD86, and human leukocyte antigen-DR cells remained similar to that of nonactivated cells. Moreover, these cells had an impaired capacity to stimulate allogeneic T lymphocytes. We demonstrated that DC maturation was altered in HTLV-1– infected patients, which could contribute to the development of HTLV-1–associated diseases.