Objective: To compare the polymorphonuclear leukocyte (PMN) functions ( phagocytosis and intracellular killing activity) of elderly patients with healthy young volunteers. Subjects and Methods: Fifty-nine elderly patients who had various diseases ( cancer, hypertension and diabetes mellitus, DM) and 10 healthy young volunteers were included in this study. Ficoll-Hypaque gradient centrifugation was used to isolate PMNs from venous blood containing EDTA (0.1 g/ml). Phagocytosis and intracellular killing activity of neutrophils were assayed using a modification of Alexander's method, in which serum opsonins, number of neutrophils and number of microorganisms are standardized in order to detect both increases and decreases in phagocytosis and intracellular killing as well as combined abnormalities of these two functions. The least significant difference test was used to compare the results in the two groups. Results: Phagocytic activity of PMNs from patients with cancer was significantly higher than that of healthy young volunteers ( p < 0.05) and elderly patients with hypertension and DM ( p < 0.05). There was no statistical difference between the phagocytic activity of PMNs from elderly patients with hypertension and DM and healthy young volunteers ( p > 0.05). The intracellular killing activity of PMNs from elderly patients with hypertension, DM and cancer was significantly lower than that of healthy young volunteers ( p = 0.001, p < 0.0001, p = 0.003, respectively). Conclusions: The intracellular killing activity of PMNs from elderly patients was significantly decreased when compared with that of healthy young volunteers. Ageing, chronic diseases and drugs used in the treatment of these elderly patients may be the cause for decreased intracellular killing activity. Copyright (C) 2005 S. Karger AG, Basel.