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Background: Definitive diagnosis of pediatric ulcerative colitis (UC) may be particularly challenging since isolated colitis with overlapping features is common in pediatric Crohn's disease (CD), while atypical phenotypes of UC are not uncommon. The Paris classification allows more accurate phenotyping of atypical inflammatory bowel disease (IBD) patients. Our aim was to identify the prevalence of atypical disease patterns in new-onset pediatric UC using the Paris classification., Methods: Information was collected from the EUROKIDS Registry, an inception cohort of untreated pediatric IBD patients undergoing evaluation at diagnosis. Patients with IBD-unclassified were excluded. Patients with isolated Crohn's colitis served as a control group., Results: Data from 898 pediatric patients (643 UC, 255 CD colitis) were included. Extensive or pancolitis was present in 77% of UC patients and macroscopic rectal sparing in 5%. Rectal sparing was inversely associated with age (mean age with rectal sparing 9.9 years vs. 11.8 without; P = 0.02). Upper gastrointestinal (UGI) involvement occurred in 4% of patients. Erosions in the stomach were present in 3.1% of children, but frank ulcerations in 0.4%; 0.8% of children had erosions or ulcerations limited to the esophagus or duodenum. The corresponding UGI involvement in Crohn's colitis was 22%. A cecal patch occurred in 2% of patients., Conclusions: Extensive disease and rectal sparing are age-dependent phenotypes in pediatric UC. Rectal sparing, cecal patch, backwash ileitis, and gastric erosions are not uncommon at diagnosis, while gastric ulcerations and erosions in the duodenum or esophagus are. Recognition of atypical phenotypes in pediatric-onset UC is crucial to prevent misclassification of IBD.