Crohn's disease and ulcerative colitis are chronic inflammatory bowel diseases (IBD) of unknown etiology. They are characterized by an activation of intestinal mononuclear cells. Cytokines play a crucial role in the regulation of the functions of these cells. An increased synthesis of the cytokines interleukin-1 (IL-1), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF alpha), which are primarily synthesized by activated monocytes/macrophages has been described in patients with IBD. The synthesis of interleukin-2 (IL-2) and of interferon gamma (IFN gamma), which are produced by lymphocytes, on the other hand, has been found to be decreased. The published data are, however, not quite consistent. In patients with IBD there is not only a stimulation of the local cytokine production in the gut. The blood levels and the synthesis of the cytokines IL-1, IL-6 and TNF alpha by peripheral blood mononuclear cells are also increased, in particular in patients with Crohn's disease. Drugs, which are commonly used for the treatment of IBD impair the synthesis of these cytokines in monocytes/macrophages.