OBJECTIVES: Gastroscopy is used to examine the upper gastrointestinal (GI) tract, but no validated method yet exists to assess the quality of mucosal visualization in children. Utilizing validated endoscopic scales can enhance study quality and standardization across centers. This study aimed to validate the existing Toronto Upper Gastrointestinal Cleaning Score (TUGCS) in pediatric patients. METHODS: This was a multicenter, prospective, single-masked study conducted in 10 European pediatric gastroenterology centers. Endoscopists with varying degrees of experience assessed the quality of mucosal visualization in prerecorded gastroscopies using the TUGCS. Each endoscopist assessed the studies two times in random order, with an interval of at least 2 weeks. The correlations of individual and total scores were statistically compared between themselves, between assessors, and between assessment attempts. Internal consistency was also checked with Cronbach's α. RESULTS: Seventeen endoscopists participated in the study. The TUGCS demonstrated high consistency within raters, with a score of 0.64 (95% confidence interval [CI]: 0.34-0.84), and an excellent test-retest reliability of 0.97 (95% CI: 0.94-0.99). The scale also showed high internal consistency, with a Cronbach's α of 0.95. The correlation between different items ranged from 0.60 to 0.77, and the correlation between individual items and the total score ranged from 0.66 to 0.88. No significant differences in the assessment were found based on the raters' experience performing endoscopy, specialization, age, or gender. The endoscopists found TUGCS easy to learn and potentially useful, especially in clinical trials. CONCLUSIONS: The TUGCS was demonstrated as a reliable and validated method for assessing the visualization quality of the upper GI mucosa in pediatric patients.