Linoleic acid (LA) and α-linolenic acid (ALA) are essential fatty acids that play an important role in modulation of T cell proliferation. The effects of consuming novel soybean oils varying in LA:ALA ratios on T cell proliferation and inflammatory responses were assessed in older adults. Eighteen participants (>50 y old) with elevated cholesterol concentrations (3.37-4.14 mmol/L LDL cholesterol) consumed 5 experimental diets in random order for periods of 35 d. Each diet contained 30% of energy as fat, two-thirds of which was high-oleic acid soybean oil (HiOleic-SO), soybean oil (SO), low-SFA soybean oil (LoSFA-SO), hydrogenated soybean oil (Hydrog-SO), or low-ALA soybean oil (LoALA-SO), resulting in LA:ALA ratios of 2.98, 8.70, 9.69, 15.2, and 18.3, respectively. Participants had higher proliferative responses to phytohemagglutinin (PHA) compared with baseline following consumption of SO (26%; P < 0.05), LoSFA-SO (22%; P < 0.05), or HiOleic-SO (24%; P < 0.05) diets. Proliferative response was similar to the baseline after participants consumed diets with an LA:ALA ratio >10 (Hydrog-SO and LoALA-SO). Post-diet intervention, LA:ALA ratios correlated with proliferative responses to PHA (r = -0.87; P = 0.05). An optimal proliferative response was observed at an LA:ALA ratio of 8.70, with an inverse correlation between proliferative response and LA:ALA ratios >8.70. These effects were independent of changes in the production of PGE(2), inflammatory cytokines, or cytokines involved in growth of lymphocytes. These data suggest that the LA:ALA ratio modulates the proliferative ability of T lymphocytes, which may be due to subtle changes in fatty acid composition of the phospholipids in immune cells.