By the use of flow cytometry, 17 patients with chronic granulomatous disease (CGD) were examined for lymphocyte subsets. CD4⁺CD29⁺ cell (memory T cell) and CD8⁺CD11b⁺ cell (suppressor T cell) subsets in CGD were significantly decreased and remained practically unchanged throughout the period examined except in those <6 months of age, although in controls both subsets gradually increased with age. These data indicate a certain abnormality in the maturation process ofCGD T lymphocytes. Memory T cells are known to secrete a large amount of interferon-γ (IFN-γ). Recently, administration of IFN-γ to CGD patients has become an effective treatment that reduces the frequency of serious bacterial infections. Decreased memory T cells may clarify the mechanism of therapeutic effectiveness, which remains unknown.