Many score methods have been created to measure paranasal sinus abnormalities seen under CT scan. Currently, the Lund-Mackay staging system is widely accepted. However, its results may be affected by the development in children. Aim: To assess the precision and accuracy of a new tomography score, called "opacification-development ratio". It translates the percentage of sinus area that is opaque. Materials and Methods: A cross-sectional study was prospectively conducted in patients ranging from 0-18 years of age who underwent CT scan assessment of rhinosinusitis. Two independent radiologists examined each scan twice, using both the Lund system and the ratio herein proposed. Results: The opacification-development ratio reached substantial intra and inter-examiner agreement, similar to the Lund system (Kappa > 0.60). Considering the Lund system as the gold standard, the most accurate cut-off point was approximately 15 (sensitivity and specificity approach 90%). 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