A dysregulated local immune reaction with unbalanced cytokine expression seems essential in inflammatory bowel disease (IBD), i.e. ulcerative colitis (UC) and Crohn's disease (CD). Since the roles of interleukin (IL-)13 and IL-15 remain unclear, this study aimed at studying intestinal expression of IL-13 and IL-15 in IBD. Methods: In colonic biopsies from 24 UC, 18 CD, and 12 controls IL-13 and IL-15 were measured using ELISA, and their gene expressions were assessed by RT-PCR. Leukocytes were visualised histochemically. Results: Concentrations of IL-13 were decreased in UC (median 56 pg/mg tissue; interquartile range 30-99 pg/mg) compared to CD (82 pg/mg tissue; 41-122; P=0.004) and controls (83 pg/mg tissue; 18-134; P>0.05), and lower in active UC (53 pg/mg tissue; 33-96) than in inactive UC (80 pg/mg tissue; 65-99; P=0.02). IL-15 concentrations were higher in CD patients (34. pg/mg tissue; 24-53) as compared to controls (20 pg/mg tissue; 15-21; P=0.001) whilst being 22 pg/mg tissue (15-32) in UC. IL-13 mRNA and IL-15 mRNA were detected in 20% and 15%, respectively. Infiltration of leukocytes correlated inversely with IL-13 levels (P=0.02). Conclusion: Active UC is associated with decreased colonic IL-13 suggesting that IL-13 levels are diminished as a part of UC exacerbations, or that exacerbations follow active downregulation of IL-13. (C) 2000 Academic Press.