Immunodeficiency with a thymoma (Good's syndrome) is a rare condition occurring in 7% to 13% of patients with adult-onset hypogammaglobulinemia. In 80% of cases, hypogammaglobulinemia is detected within 5 years of the identification of the thymoma. A 70-year-old man was found to have hypogammaglobulinemia 18 years after a thymoma had been resected. Cellular immunophenotyping revealed there were no detectable B cells, decreased CD4+ cells, and increased CD8+ cells. Both CD4+ and CD8+ subsets expressed increased populations of CD38+ DR+ cells and CD45RO+ cells. The CD8+ CD28+ population was markedly reduced. Inducible cytokine production by the patient's peripheral blood mononuclear cells revealed decreased IL-2, IL-10, and interferon-gamma production. These data suggest that patients with Good's syndrome have activated memory T cells that have dysregulated cytokine production.