Highly reactive oxygen species (ROS) are involved in T-cell activation and in the defense against environmental pathogens. An imbalance of ROS generation, detoxifying scavenger enzymes, and molecules with antioxidant capacity could contribute to the increased susceptibility to cancer and infections in severe humoral immunodeficiency. We studied antioxidant status, i.e., plasma antioxidant capacity (TEAC), retinol, alpha-to-copherol, ubiquinol, and the number of activated T cells in 16 patients with common variable immunodeficiency (CVID) compared to age-matched healthy controls. As expected, patients showed significantly increased levels of activated HLA-DR and CD45RO-expressing T cells. Plasma levels of the endogenous ROS scavenger ubiquinol (Q 10) were significantly lower in patients as compared to controls. However, patients showed only slightly reduced levels of TEAC as well as the exogenous antioxidants retinol and alpha-tocopherol. Although no correlation of the number of activated T cells and antioxidant capacity could be demonstrated, an increase in ROS and a diminished reactive oxygen scavenger capacity may be involved in the disease process in patients with common variable immunodeficiency.