Objectives: To investigate the appearance of the pediatric larynx on ultrasound images and the diagnostic potential of ultrasound in cases of pediatric vocal fold paralysis. Methods: First, we confirmed the ultrasonographic features of the laryngeal landmarks in 2 pediatric cadaveric larynxes. Secondly, 45 children were enrolled in a clinical study (13 patients with vocal fold paralysis and 32 normal children). Quantitative analysis of vocal fold mobility was carried out by measuring the maximum glottic angle (MGA) and vocal fold-arytenoid angle (VAA). Results: All the paralyzed vocal folds showed abnormal mobility, and were flaccid during breathing and phonation. The rima glottis appeared as a hyperechoic air-column band in ultrasound images during phonation. The mean value of the MGA was 61.47 +/- 9.00 in the normal larynx and 42.25 +/- 10.41 in the paralyzed larynx. In the affected side of the paralyzed larynx, the VAA in maximum abduction was smaller than that in the normal larynx or in the unaffected side. The median difference of the VAA between maximum abduction and maximum adduction was less than that in the normal larynx. The kappa value was 0.96. Conclusion: MGA and VAA are quantitative indicators of vocal fold immobility. Ultrasound is a reliable method of diagnosis of pediatric VFP. To diagnose VFP from an ultrasound image, the criteria are: (1) abnormal mobility (this was the most important and direct evidence), (2) hyperechoic air-column band of the glottic rima during phonation, (3) flaccid vocal fold and (4) asymmetry of the glottal structures. (C) 2011 Elsevier Ireland Ltd. All rights reserved.