Glutamine, a conditionally essential amino acid, is important for immune function. It is now being formulated for incorporation into total parenteral nutrition (TPN). The aims of this study were to examine the effect of glutamine administration on lymphocyte proliferation and proinflammatory cytokine release in patients with severe acute pancreatitis. Fourteen patients were randomized (in a double-blind fashion) to receive either conventional or isocaloric, isonitrogenous glutamine-supplemented (0.22 g glutamine x kg(-1) x d(-1) as glycyl-glutamine) TPN for 7 d. DNA synthesis (index of lymphocyte proliferation) and the 24-h release of tumor necrosis factor (TNF), interleukin (IL)-6, and IL-8 from peripheral blood mononuclear cells were measured in vitro on days 0, 4, and 7. Thirteen patients completed the study protocol (6 glutamine TPN, 7 conventional TPN). Glutamine supplementation increased median DNA synthesis by 3099 cpm over the study period against 219 cpm in the conventional group (increase not significantly different between the two groups) . Glutamine supplementation did not significantly influence TNF or IL-6 release, but, in contrast, median IL-8 release was reduced by day 7 in the glutamine group while it was increased in the conventional group (-17.7 ng/mL (median change over study period) versus +43.3 ng/mL, respectively; P=0.045). Small patient numbers and substantial interindividual variation limit the conclusions, but there is a trend for the glutamine group to have improved lymphocyte proliferation, and in the case of IL-8, reduced proinflammatory cytokine release.