JC virus, the cause of progressive multifocal leukoencephalopathy (PML), and the BK virus are genetically similar and share sequence homology in immunogenic proteins. We treated three immunosuppressed patients with PML with ex vivo–expanded, partially HLA-matched, third-party–produced, cryopreserved BK virus–specific T cells. The immunosuppression in these patients was due to the conditioning regimen for cord-blood transplantation in one patient, a myeloproliferative neoplasm treated with ruxolitinib in another, and acquired immunodeficiency syndrome in the third. After T-cell infusion in two of the patients, alleviation of the clinical signs and imaging features of PML was seen and JC virus in the cerebrospinal fluid (CSF) cleared. The other patient had a reduction in JC viral load and stabilization of symptoms that persisted until her death 8 months after the first infusion. Two of the patients had immune reconstitution syndrome. Donor-derived T cells were detected in the CSF after infusion. (Funded by the M.D. Anderson Cancer Center Moon Shots Program and the National Institutes of Health; ClinicalTrials.gov number, NCT02479698.)