['Natural killer (NK) cells are cytotoxic lymphocytes that lack CD3 and express variable levels of CD16, CD56 and CD57. In recent years NK cells have been categorised into two major groups based on the level of CD56 expression. This phenotypic classification correlates with functional activity as CD56bright NK cells are the major cytokine producing subset whereas CD56dim NK cells exhibit greater cytotoxic activity. Previous studies have revealed a reduction in total NK cell numbers in association with ageing and this study sought to determine the potential influence of ageing on the number of NK cell subsets within peripheral blood.', 'The number of NK (CD56+CD3-) cells within peripheral blood did not change with increasing age. The number of CD56dim NK cells also remained stable with ageing. In contrast the absolute number of CD56bright NK cells within peripheral blood declined by 48% with ageing from a mean of 15.6/microl in individuals aged 20-40 years to 8.1/microl in those aged 60+ years (p = 0.0004).'] Is ageing associated with a decline in peripheral blood CD56bright NK cells?