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Objectives: T cells play a crucial role in many chronic inflammatory diseases. Mucosal T cells are particularly important in the pathogenesis of Crohn's disease (CD). We investigated the response of T cells in CD and other intestinal inflammatory conditions to interleukin-2 (IL-2), a cytokine essential for T-cell activation, growth, and function. Study design: T-cell reactivity was assessed by measuring growth induced by IL-2 in mucosal endoscopic biopsy specimens obtained from children with CD, ulcerative colitis, indeterminate colitis, and chronic nonspecific colitis and from children without gastrointestinal inflammation. Results: CD mucosal T cells grew remarkably and significantly more than T cells from normal, ulcerative colitis, and chronic nonspecific colitis mucosa. T cells from indeterminate colitis mucosa grew similarly to those of CD mucosa. The enhanced growth response in CD was independent of disease location, presence or absence of intestinal inflammation, treatment, disease duration, or clinical activity. Conclusion: Mucosal T cells from children with CD exhibit an intrinsic hyperreactivity to IL-2. This may represent a primary pathogenic abnormality in this condition. (J Pediatr 1998;133:675-81)