Aging is a chronic oxidation process in which the immune system is involved. Because leukocyte functions is a good health marker and longevity predictor, the effects of daily oral administration of N-acetylcysteine (NAC, 600 mg) on several lymphocyte (adherence, chemotaxis, proliferation, natural killer activity) and neutrophil (adherence, chemotaxis, phagocytosis, superoxide) functions, as well as cytokine levels (interleukin-2, tumor necrosis factor alpha, interleukin-8), were studied in 36 healthy postmenopausal women: 18 aged 50-69 years and 18 aged > 69 years. In addition, plasma and leukocyte oxidative stress markers (glutathione, superoxide, malondialdehyde) were evaluated. These parameters were analyzed within 2 and 4 months of of NAC intake and 3 months after the end of the supplementation. In parallel, samples from 18 healthy adult women aged 30-49 years were used as a control age group. the results showed general impairment of immune function and increased oxidation markers in postmenopausal women as compared with the control group; however, NAC administration significantly improved the parameters studied, bringing their values closer to those of younger women and thus exerting a modulatory, rather than a merely stimulatory, action on the immune system. These effects were also observed 3 months after the end of supplementation. The present finding suggest that a short period of NAC supply (i.e., 2-4 months) at the dose used may lead to prolonged strengthening of immune defence in postmenopausal women, likely by increasing the leukocyte glutathione pool. Thus, NAC could contribute to maintenance of good health and quality of life in postmenopausal women by decreasing the probability of immune system-related diseases, such as infections, in aging.