In immune-competent individuals, human cytomegalovirus (HCMV) infection is associated with impairment of T-cell function. Our goal was to evaluate prospectively whether clinically asymptomatic HCMV infection in allogeneic hematopoietic stem cell transplantation (alloHSCT) recipients, treated pre emptively with ganciclovir, influences T-cell function as well. Mitogen-stimulated T-cell proliferative activity, together with cell surface markers, was tested in 49 patients on days + 30, + 45, + 60, and + 90 after alloHSCT and, additionally, in cases of positive HCMV pp65-antigenemia. HCMV infection was diagnosed in 19 patients. None of them developed HCMV disease. T-cell proliferative activity was significantly decreased on days when HCMV antigenemia was positive as compared to days without antigenemia. The number of pp65-positive cells negatively correlated with proliferative response. Comparison of patients who did experience HCMV infection with those who did not reveals significant decrease of T-cell proliferative activity observed on days + 30 and + 45, a time period when antigenemia was most frequently found to be positive, whereas no difference was detected on days + 60 and + 90. We conclude that, even clinically asymptomatic, HCMV infection has negative impact on T-cell proliferation capacity in alloHSCT recipients. However, pre emptive therapy with ganciclovir makes this immunosuppressive effect transient and restricted to the time of infection duration.