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Background and Aims: Endoscopy and histopathology are pivotal for evaluating disease activity in ulcerative colitis [UC]; correlation between validated endoscopic and histological indices has not been examined. We aim to correlate the Ulcerative Colitis Endoscopic Index of Severity [UCEIS] with two new validated histological indices in patients with established UC., Methods: This was a single-centre cohort of patients with established UC, who underwent flexible sigmoidoscopy or colonoscopy by a single endoscopist. The UCEIS was scored at the worst affected area in the distal colon, which was biopsied; histological disease activity using Nancy [NI] and Robarts' Histological [RHI] indices was scored by a pathologist blinded to the endoscopy. Spearman correlation between the UCEIS, NI, and RHI, and between NI and RHI, was performed., Results: A total of 125 patients, median age 37 years [range 16-81 years], with UCEIS scores [scale 0-8]: 0, n = 21; 1-3, n = 48; 4-6, n = 51; and 7-8, n = 5, were included. Correlation coefficients between UCEIS and NI [scale 0-4] were r = 0.84 (95% confidence interval [CI] 0.76-0.89, p < 0.001) and between UCEIS and RHI [scale 0-33] r = 0.86 [95% CI 0.80-0.90, p < 0.001]. The difference in correlation was not significant [p = 0.57]. There was excellent correlation between the two histological indices [r = 0.92, 95% CI 0.87-0.95, p < 0.001]. Quiescent disease activity defined as the absence of neutrophils [Nancy 0-1, Robarts 0-3] was most closely correlated with UCEIS = 0., Conclusions: The UCEIS strongly correlates with both NI and RHI. Complete mucosal healing is best defined as a UCEIS = 0/8, since this correlates with the absence of microscopic disease activity.