Introduction: Ulcerative colitis (UC) is a chronic disease characterized by periods of inflammatory activity and remission, which vary from the rectum to the proximal colon. Currently, mucosal healing is a long-term goal in the management of inflammatory bowel disease, with colonoscopy and sigmoidoscopy being the recommended tools for evaluation. Objective: To assess the effectiveness of both examinations in determining the presence of inflammatory activity in the follow-up of patients with UC. Methods: Retrospective observational study analyzing colonoscopies performed as part of the follow-up of UC patients between January 2021 and July 2023 by gastroenterologists from the Inflammatory Bowel Disease Program at the Clínica Universidad de los Andes. The study compared endoscopic and histological activity observed in the rectosigmoid region with that found in the rest of the colon. Sensitivity and specificity were determined using concordance and correlations tests. Results: A very good concordance and correlation were observed regarding endoscopic findings, with a Kappa index of 0.97 and a Spearman coefficient of 0.97. The Positive Predictive Value (PPV) of sigmoidoscopy for endoscopic activity was 1, and the Negative Predictive Value (NPV) was 0.96. In relation to histological activity, the concordance had a Kappa index of 0.93 and a Spearman coefficient of 0.93, with a PPV of sigmoidoscopy for histological activity being 1 and an NPV of 0.91. Conclusion: This cohort suggests that sigmoidoscopy is a cost-effective option for evaluating mucosal healing in UC patients in symptomatic and biomarker remission. However, complete colonoscopy should be considered in cases of discrepancies with the clinical picture or in colorectal cancer surveillance.