The risk of colorectal carcinoma is increased inpediatric-onset inflammatory bowel disease (IBD). Thereis little information available regarding the colonicmucosal proliferative state in children with IBD. The aim of this study was to assesscolonic ornithine decarboxylase (ODC) activity, a markerof cell proliferation, in pediatric IBD patients. ODCactivity was assessed in colonic mucosa from 23 children (7 with ulcerative colitis, 9 with Crohn'sdisease, and 7 controls) undergoing colonoscopicexamination. ODC activities were then compared withdegree of inflammation of biopsied samples. ODCactivities in patients with and without corticosteroidtreatment were also analyzed. The mucosal ODC activitiesof sigmoid colon and rectum were significantly higher(2.5- to 4-fold) in both ulcerative colitis and Crohn's disease. The higher ODC activity wasassociated with increased mucosal inflammation.Moreover, treatment with corticosteroids decreased theODC activity. In conclusion, using ODC activities as a marker of cell proliferation, our resultssuggest that there is a higher colonic mucosalproliferative state in children with IBD. The increasedODC activities were associated with increased colonicmucosal inflammation. Colonic mucosal ODC activity mayprovide an additional parameter to access thetherapeutic efficacy of corticosteroid treatment inpediatric IBD patients.