Background: The aim of this study was to evaluate diaphragmatic parameters in bronchiolitis patients and identify correlations between clinical and sonographic severity scores and outcomes in order to develop a more objective and useful tool in the emergency department. Methods: Children aged between 1 and 24 months and diagnosed with acute bronchiolitis were included in the study. The Modified Respiratory Distress Assessment Instrument (mRDAI) score was used to quantify the clinical severity of the disease. Lung ultrasound was performed and a bronchiolitis ultrasound score (BUS) was calculated. Diaphragm ultrasound was then performed and diaphragm thickness at the end of inspiration and expiration, thickening fraction, diaphragm excursion (EXC), inspiratory slope (IS), expiratory slope (ES), and total duration time of the respiratory cycle were measured. Results: There were 104 patients evaluated in this study. The mRDAI score and BUS had a significant positive correlation. There was a positive correlation between IS and respiratory rate at admission. As the clinical score increased, IS, ES, and EXC measurements rose and they were positively correlated. Values of IS, ES, and EXC were higher in the moderate-severe group than the mild group for both mRDAI and BUS scores. Inspiratory slope values were correlated with the length of stay in the hospital. Conclusion: Values of IS and ES were correlated with clinical and sonographic severity scores. Moreover, IS was a good predictor of outcome. Diaphragm ultrasound appears to be an objective and useful tool to help the physician make decisions regarding the evaluation and management of bronchiolitis.