Crohn's disease (CD) is a chronic bowel inflammatory disorder in which the pathogenic role of immune alterations has been suggested, but the immunologic mechanisms responsible for the inflammatory reaction are still poorly understood. We investigated the profile of cytokine secretion by T-cell clones generated from gut tissue specimens of four patients with active CD, five patients with ulcerative colitis, and four patients with noninflammatory gut disorders (NIGDs). The great majority of CD4+ T-cell clones generated from the gut of patients with CD produced high levels of interferon-gamma (IFN-gamma) but low or undetectable amounts of interleukin-4 (IL-4), whereas substantial proportions of CD4+ T-cell clones derived from the gut of patients with either ulcerative colitis or NIGDs produced IL-4 in addition to IFN-gamma. The immunohistochemical analysis revealed high numbers of activated CD4+ T cells showing IFN-gamma but not IL-4 reactivity, as well as substantial proportions of IL-12-containing macrophages, in the intestinal lamina propria and muscularis propria of patients with CD, whereas these cells were very rare or undetectable in patients with NIGDs. Culturing T cells from gut biopsy specimens of a patient with CD in the presence of a neutralizing anti-IL-12 antibody down-regulated the development of IFN-gamma-producing CD4+ T cells. These findings suggest that a critical event in the initiation of bowel inflammatory lesions in CD may involve up-regulation of IL-12 production, resulting in conditions that maximally promote type 1 T-helper immune responses.