In total, 52 infants were included with a median age of 19.5 (0-70) weeks, with 12 and 40 infants, respectively, for the assessment of the laryngeal anatomic landmarks and the assessment of the RFS-I. Overall interobserver agreement of the RFS-I was moderate (intraclass correlation coefficient = 0.45). Intraobserver agreement ranged from moderate to excellent agreement (intraclass correlation coefficient = 0.50-0.87). A standardized scoring instrument was developed for the diagnosis of LPR disease using flexible laryngoscopy. Using this tool, only moderate interobserver agreement was reached with a highly variable intraobserver agreement.