Leukotriene B₄, an arachidonic acid metabolite, is a potent chemotactic agent, and is thought to be an important mediator of inflammation. To investigate the role of this compound as a mediator of inflammation in inflammatory bowel disease, arachidonic acid was incubated with ionophore and colonic mucosa from patients with inflammatory bowel disease and from normal subjects. Mucosa from patients with inflammatory bowel disease converted 2.17% of exogenous arachidonate to leukotriene B₄; mucosa from normal subjects converted 0.37%. The production of leukotriene was blocked by sulfasalazine. To determine if inflammatory bowel mucosa contained endogenous leukotriene B₄, lipid extracts were analyzed by high pressure liquid chromatography. Mucosa from patients with inflammatory bowel disease contained 254 ng of leukotriene B₄ per gram and mucosa from normal subjects contained < 5 ng of leukotriene B₄ per gram. The presence of significant amounts of leukotriene B₄ in colonic mucosa in patients with inflammatory bowel disease, combined with the known biologic effects of leukotriene B₄, suggests that it may be an important mediator of inflammation in inflammatory bowel disease.