Abstract Background Inflammatory bowel diseases (IBD) are diagnosed during childhood and adolescence in up to 20% of patients. Invasive studies are mandatory for diagnosis, but monitoring strategies should ideally be non-invasive to reduce anxiety and stress, especially in children. Hence, intestinal ultrasound (IUS) is increasingly being used as a cheap, noninvasive tool for monitoring disease activity in children with IBD. However, scoring systems have mostly been developed and validated for adults. We aimed to evaluate the performance of the Milan Ultrasound Criteria (MUC) and simple pediatric activity ultrasound score (SPAUSS) in pediatric ulcerative colitis (UC) patients, and correlate them with the PUCAI (Pediatric Ulcerative Colitis Activity Index) score. Methods Patients <18 years with an established diagnosis of UCwere prospectively enrolled in Colombia. IUS assessment was performed (including measurement of Bowel Wall Thickness and Colour Doppler Sign), with subsequent collection of Mayo endoscopic score (MES) and PUCAI scores (The information was recorded with data from a colonoscopy performed ± 30 days). Appropriate uni- and bivariate analyses were performed, as well as Pearson correlation. Results Sixteen ambulatory pediatric patients with UC (50.0% girls median age 13.3 years [9-17], median disease duration 5.1 years [0.7-13.7]) were enrolled. The PUCAI score has a significant statistical relationship with MES (p=0.001) and the MUC (p=0.001). Likewise, the PUCAI score was associated with the presence of a Doppler signal (p=0.001). The SPAUSS score correlated strongly with the MUC score (r=0.938), (p=0.001). The median MUC score in children with MES 0-1 versus MES≥2 was 3.08 and 6.87 respectively. Based on additional ROC analysis (Figure 1), we propose a MUC cutoff of 4.85 with 100% sensitivity and specificity to differentiate MES 0-1 from MES≥2 (p=0.003) (Figure 2). Conclusion The MUC score in pediatric UC patients correlates well with endoscopic disease activity and with the SPAUSS score. We propose a MUC score cutoff of 4.85 for pediatric UC patients. Larger, independent studies are needed to confirm this cutoff, which is different to the MUC 6.2 cutoff in adult UC patients.