In the spinal cord of patients with human T cell leukemia virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP), infiltrating CD4(+) lymphocytes seem to be the major reservoir for the virus. Little, however, is known about the mechanisms by which HTLV-1 crosses the blood-brain barrier. An oligoclonal proliferation of HTLV-1-infected CD4 lymphoid T cells is present in the peripheral blood of all HTLV-1-infected individuals. Here, such oligoclonal distribution of HTLV-1-infected cells is evidenced in the cerebrospinal fluid (CSF) derived from 5 patients with HAM/TSP. Furthermore, clonal populations of HTLV-1-infected lymphocytes sharing the same HTLV-1 proviral flanking sequences (i.e. , integration sites in the cellular DNA), and thus derived from a single HTLV-1-infected progenitor, were found, for a given patient, in both the CSF and the peripheral blood. These data demonstrate that HTLV-1 crosses the blood-brain barrier by migration of HTLV-1-infected lymphocytes in vivo.