Although low plasma vitamin A concentrations are associated with increased incidence or severity of infections such as respiratory tract infection and measles in children, there is a paucity of data on the effect of vitamin A deficiency on the distribution of, and cytokine production by, the different cellular immune subsets in humans. We conducted a cross-sectional study in a district in Ghana to characterize cellular subsets and functional capacity of peripheral blood mononuclear cells from vitamin A-deficient and vitamin A-sufficient (normal) individuals and evaluated the relationships between vitamin A concentration in plasma and cellular immune status. We measured the percentages of selected cellular phenotypes and intracellular cytokine expression and describe the differential cellular subset distributions and alterations in cytokine expression in participants with normal and deficient vitamin A concentrations. The major change observed in the constitution of cellular subsets was a decrease in tumor necrosis factor α expressing CD3-CD56+ natural killer (NK) cells in those with vitamin A deficiency, compared with normal individuals. CD4+ T-cell proliferation and production of interferon γ and interleukin 4 were not statistically different between the 2 groups. These results support previous studies that demonstrated decreased NK cell activity in vitamin A-deficient animals. The decrease in tumor necrosis factor α expressing NK cells observed in vitamin A-deficient individuals in this study could help to explain the decreased resistance to infections observed in those with vitamin A deficiency. © 2007 Elsevier Inc. All rights reserved.