We characterized inflammatory infiltrates in archival brain biopsy and autopsy samples from 26 HIV(+) and 20 HIV(-) patients with progressive multifocal leukoencephalopathy (PML). The predominant inflammatory cells were CD8(+) T lymphocytes. We defined CD8(+) T cell distribution with regard to JCV-infected glial cells, PML lesions and the extent of demyelination. In most samples from either HIV(+) and HIV(-) patients, we found positive correlations between the parenchymal CD8(+) T cells and JCV-infected glial cells and conversely, negative correlations between the perivascular CD8(+) T cells and JCV-infected glial cells in the surrounding brain. Most of these correlations remained significant after accounting for the degree of demyelination and location of the cells relative to lesions. Moreover, high numbers of CD8(+) T cells were found within and at the border of active PML lesions. These results suggest that CD8(+) T cells are primarily associated with JCV-infected glial cells in most PML cases and that an active ongoing recruitment of CD8(+) T cells and possibly viral antigen-specific retention could occur. These observations are discussed in the context of the recent evidence of PML in multiple sclerosis and Crohn's patients treated with natalizumab, underscoring the role of CD8(+) T lymphocytes in continued immunosurveillance of the CNS.