Introduction The prevalence of pediatric-onset inflammatory bowel disease ( PIBD) has been dramatically increased in Korea. The revised Porto criteria was proposed to standardize the PIBD subtypes and has provided a clear definition of ulcerative colitis (UC), atypical UC, Crohn’s disease (CD), and inflammatory bowel disease unspecified (IBDU), which had used to be subjective. Aims This study aims to evaluate a change of diagnosis from originally diagnosed UC to atypical UC, CD, and IBDU respectively in Korea by using the revised Porto criteria. Material and methods We retrospectively reviewed the medical record of 211 UC who had been diagnosed before the age of 18 by PIBD experts according to the accepted diagnostic criteria at the time of diagnosis from 2005 to 2013. Each data was categorized by its baseline and follow-up disease characteristics based on endoscopic and histologic findings. Results Among total of 211 patients, the data of 80 UC patients, which had been collected at the time of initial diagnosis, were eligible to evaluate the change of diagnosis based on the Porto criteria. While 39/80 (48.75%) of UC children maintained their original diagnosis, 41/80 (51.25%) of UC patients were reclassified to IBDU and atypical UC. Conclusions The Porto criteria suggested that the prevalence of IBDU and atypical UC could be greater than that of typical UC among those who had been previously diagnosed as pediatric UC in Korea. Given the notion of atypical UC and IBDU as a true intermediate phenotype of PIBD, clinicians should consider multiple variables of the criteria and undergo complete endoscopic and histologic work-up at the time of diagnosis with repeated investigation for every PIBD patient.