The associations between platelet fatty acids and peripheral blood lymphocyte subsets were studied in 78 institutionalized elderly individuals (27 men and 51 women), aged 67 to 100. Platelet fatty acids were assessed by gas chromatography, and peripheral blood lymphocyte subsets were quantitated by immunophenotyping using flow cytometry. It was found that women had a higher number of total T-cells (CD3), T-helper (CD3+4+) cells, and B-cells (CD19). However, no gender differences were observed in the percentages of lymphocyte subsets. In elderly men, after adjusting for age and fatty acid intake, the platelet concentration of omega-3 polyunsaturated fatty acids was positively related to the percentage of CD3 and CD3+4+ bearing lymphocytes (rs = 0.59, P < 0.05; and rs = 0.55, P < 0.05, respectively), and the concentration of total saturated fatty acids was also positively associated with the percentage of B (CD19) cells (rs = 0.50, P < 0.05). However, similar relationships were not observed in elderly women. No significant associations were found between trans fatty acids and any of the lymphocyte subsets in the study population. These findings suggest that fatty acids may be related to immune function. Any effects may be important in the host immune defence, especially in elderly individuals.