Purpose: To develop a predictive score for small diaphragmatic defects in infants with congenital diaphragmatic hernia (CDH).Methods: The Japanese CDH Study Group cohort was randomly divided into derivation (n = 397) and validation (n = 396) datasets. Using logistic regression, a prediction model and weighted scoring system for small diaphragmatic defects were created from derivation dataset and validated with validation dataset.Results: Six weighted variables were selected: no hydramnios, 1 point; 1-minute Apgar score of 5-10, 1 point; apex type of the lung (left lung is detected radiographically in apex area), 1 point; oxygenation index < 8, 1 point; abdominal nasogastric tube (tip of the nasogastric tube is detected radiographically in abdominal area), 2 points; no right-to-left ow of ductus arteriosus, 1 point. In validation dataset, rates of small diaphragmatic defects for Possible (0-3 points), Probable (4-5 points), and De nite (6-7 points) groups were 36%, 81%, and 94%, respectively (p < 0.001). Additionally, sensitivity, speci city, positive predictive value, and C statistics were 0.78, 0.79, 0.88, 0.76, and 0.45, 0.94, 0.94, 0.70 for Probable and De nite groups, respectively. Conclusion: Our simple scoring system effectively predicted small diaphragmatic defects in infants with CDH and is useful to determine thoracoscopic surgery indication.