Decreased expression of CD158a on CD8⁺ cytotoxic T cells is related to CMV seropositivity in B-CLL patients. CMV-seronegative controls (○, n = 3), CMV-seropositive controls (•, n = 5), and CMV-seronegative B-CLL patients (□, n = 4) exhibit an elevated percentage of CD158a⁺ cells within the CD8⁺CD45RA⁺CD27^- subset compared with the respective total CD8⁺ cells, whereas the percentage of CD158a⁺ within the CD8⁺CD45RA⁺CD27^- subset of CMV-seropositive (▪, n = 12) B-CLL patients is similar for the total CD8⁺ cells and significantly lower compared with CMV-seronegative B-CLL patients (P = .0092).