More affordable and less invasive than resting endoscopy, ultrasonography of the equine larynx also provides a better correlation with dynamic endoscopy. With appropriate objective ultrasonographic suspicion criteria, its use for early screening would then allow subjecting only suspected horses to a dynamic endoscopic exam for confirmation. In this study, we tried to determine a quantitative ultrasonographic parameter that would both take concordant values between two different operators and reliably differentiate healthy horses from horses showing a decreased abduction of the left arytenoid cartilage at dynamic endoscopic evaluation. Calculated from three images in transverse section, the parameter that ensures the best inter-operator concordance is the mean ratio between mean echogenicity of a 1130 pixels disc located in the cricoarytenoideus lateralis muscle and mean echogenicity of a 1130 pixels disc located in the vocalis muscle. This parameter also provides a very good correlation with dynamic endoscopic exam, which is the gold standard.