Objective: To determine zinc status and age-related changes in the immune function of healthy late-middle-aged men and women (aged 55-70 y). Design: Observational study. Setting: Population of Northern Ireland. Subjects: Apparently healthy, free-living individuals (45 men, 48 women) aged 55-70 y. Intervention: Zinc status markers were analysed by flame atomic absorption spectrometry and commercially available kits. Immune function was assessed by flow cytometry. Results: Serum and erythrocyte zinc concentrations were 13.0 (s.d. 1.40) mu mol/l and 222 (s.d. 48.2) mu mol/l, respectively. Serum alkaline phosphatase (ALP) concentrations were 76.8 (s.d. 16.1) U/I; women showed significantly higher concentrations of ALP (P=0.011). Women demonstrated (1) a significant inverse correlation in naive T lymphocytes, specifically naive T-helper lymphocytes (% expression, r= -0.364, P=0.007 and absolute count, r= -0.275, P=0.036) with age and (2) a significant positive correlation between late activation of T lymphocytes (% expression, r=0.299, P=0.019 and absolute count, r=0.260, P=0.039) with advancing age. Men demonstrated a significant positive correlation in the % expression of (CD3-/CD16 + CD56 +) natural killer (NK) cells with age (r=0.316, P=0.017). Conclusions: Between the ages of 55 and 70 y, healthy individuals experience significant alterations in immune function; however, such changes appear largely sex specific. Given the reported importance of adequate zinc status in maintaining optimal immune function, further studies are required to explore the effect of enhanced zinc status on emerging immune deficiencies in cell-mediated immunity in healthy 55-70 y olds.