We show that at least half of patients with common variable immunodeficiency (CVID) have circulating CD8+ T cells specific for epitopes derived from cytomegalovirus (CMV) and/or the Epstein–Barr virus (EBV). Compared to healthy age-matched subjects, more CD8+ T cells in CVID patients were committed to CMV. Despite previous reports of defects in antigen presentation and cellular immunity in CVID, specific CD4+ and CD8+ T cells produced interferon (IFN)-γ after stimulation with CMV peptides, and peripheral blood mononuclear cells secreted perforin in response to these antigens. In CVID patients we found an association between a high percentage of circulating CD8+ CD57+ T cells containing perforin, CMV infection and a low CD4/CD8 ratio, suggesting that CMV may have a major role in the T cell abnormalities described previously in this disease. We also show preliminary evidence that CMV contributes to the previously unexplained severe enteropathy that occurs in about 5% of patients.