OBJECTIVE: Ulcerative colitis (UC) is an inflammatory bowel disease involving the rectum and colon. Transabdominal bowel ultrasound (TBUS) is a noninvasive technique evaluating inflamed colonic segments. Previous studies showed optimal concordance between TBUS and endoscopy in the colon, but suboptimal in the rectum. Transperineal ultrasound (TPUS) of the rectum achieved high agreement with endoscopy in adult UC patients. This study aimed to assess the accuracy of ultrasound in evaluating disease activity in pediatric UC patients through TBUS and TPUS examinations. METHODS: All pediatric UC patients who underwent endoscopy were consecutively enrolled. Disease activity was determined using the PUCAI score and the Mayo Endoscopic Score for each segment. Remission was defined as Mayo</=1 at endoscopy and bowel wall thickness </=3 mm in the colon and </=4 mm in the rectum at ultrasound examination. A concordance analysis comparing endoscopy and the US was performed overall and for each segment. RESULTS: Twenty-six patients were enrolled. Ten patients had an endoscopic remission/inactive disease; 16 had moderate-severe colitis, of whom 6 had an isolated rectal involvement, and 10 had an active disease involving segments proximal to the rectum. We showed a good performance of TBUS for all colonic segments and for the rectum, when visible; the TPUS was a feasible technique with a good concordance with endoscopic findings (Cohen kappa-value 0.77). CONCLUSION: In our pediatric population, both TBUS and TPUS showed an overall good correlation with endoscopy results and may represent a good surrogate of endoscopy, when not advisable.