BackgroundTransabdominal ultrasonography (US) has been reported to be a useful tool for evaluating ulcerative colitis (UC) although with less well-established data than for Crohn’s disease. This prospective multicenter study aimed to establish the usefulness of US compared with colonoscopy (CS) for assessing disease extent and activity of UC.MethodsAltogether, 173 patients with UC were prospectively enrolled, among whom 156 were eligible for this study. All patients underwent US and CS within 2 days at five facilities. We divided the colon into six segments and examined each segment and the rectum using US and CS. US severity was graded 1–4 regarding bowel wall thickness, stratification, and ulceration. CS severity was also graded 1–4 according to Matts’ endoscopic classification. Concordance between US and CS grades for all colonic segments was analyzed using kappa statistics. US and CS findings were also compared with the clinical disease activity index (CAI) and histological grade using Spearman's correlation coefficient.ResultsThere was moderate concordance between US and CS grades in all colonic segments (weighted κ = 0.55, p < 0.001). Concordance was rated moderate for each colonic segment but only slight for the rectum. The US grade was significantly correlated with the CAI score (r = 0.40, p < 0.001) and histological grade (r = 0.35, p < 0.001).ConclusionsThis prospective multicenter study showed moderate concordance between US and CS for assessing the disease activity of UC. Hence, US may be used more generally for evaluating UC in daily clinical practice.