The defects in T-cell function seen in CLL are present irrespective of CMV serostatus. The function of CLL CD8⁺ T cells was compared with CD8⁺ T cells from healthy controls matched for CMV serostatus. Increased production of IFNγ (A), increased expression of TBET (B), decreased proliferative capacity (C), and decreased cytolytic ability (D) of CLL T cells were all found irrespective of CMV serostatus. Pulsed target cells (closed symbols); unpulsed control target cells (open symbols). The graph representing percentage-specific lysis shows the mean and standard error of results obtained from 13 CLL patients, (5 CMV− and 8 CMV+), and 12 healthy controls (5 CMV− and 7 CMV+).