A quantitative study of the T cell receptor repertoire was performed ex vivo on CD4 and CD8 T cell subsets of human T cell leukemia virus type I (HTLV-I)–infected asymptomatic carriers and patients with HTLV-I–associated myelopathy/tropical spastic paraparesis (HAM/TSP). Indexes of oligoclonality that compiled all repertoire modifications were calculated for peripheral blood mononuclear cells and for CD4 and CD8 T cell subsets. Both patients with HAM/TSP and asymptomatic carriers had greater T lymphocyte expansions than did uninfected donors, which was independent of age and at least twice higher in the CD8 than in the CD4 cell compartment. Some expanded CD8 T cells corresponded to cytotoxic T lymphocytes directed against various epitopes of the immunodominant Tax protein. Patients with HAM/TSP had significantly higher CD8 cell expansions than did asymptomatic carriers. These results highlight the prognostic value of measuring CD8 T cell expansions during follow-up of HTLV-I infection