Crohn's disease and ulcerative colitis are chronic inflammatory diseases of the bowel with unknown etiology. Alterations of intestinal and systemic immuno-regulation are probably of importance in the pathophysiology of both diseases. Moreover, recent studies suggest that polymorphonuclear granulocytes and probably macrophages are of particular importance in the initiation of an acute flare of the disease. Interest has therefore focused on arachidonic acid metabolites, in particular leukotrienes and prostaglandins, which are important mediators in granulocyte and macrophage activation and chemotaxis. The pathophysiologic importance of various leukotriene derivatives, particularly LTB4, has been well established whereas the actions of prostaglandins in vivo, in particular PGE2, are controversial. Numerous anti-inflammatory drugs such as glucocorticoids and salicylic acid derivatives have been successfully used to treat intestinal inflammation. One possible mechanism of action is the modulation of arachidonic acid metabolism and thus regulation of the genesis of active inflammatory mediators.