BACKGROUND This study aims to investigate the role of ultrasonography in the postoperative evaluation of diaphragm function in patients with congenital heart defect. METHODS This prospective study included a total of 360 patients (176 males, 184 females; mean age 2 years; range, 1 month to 8 years) who underwent congenital heart surgery and 44 patients (22 males, 22 females; mean age 1 years; range, 1 month to 4 years) who underwent diaphragm ultrasonography between September 2018 and March 2019. Ultrasonography was performed for the patients who had difficulty in weaning from mechanical ventilation or who were thought to have diaphragm dysfunction due to pathological findings on postoperative chest X-rays. The findings were interpreted as normal, paresis, or paralysis. RESULTS Diaphragm dysfunction was demonstrated in 23 patients (6.3%), paralysis in 11 patients (3%), and paresis in 12 patients (3.3%). A median sternotomy was performed in 21 patients (91%), and seven of them (30%) were redo cases. Five patients (21%) had single ventricle physiology. Six patients (1.6%) needed an intervention due to diaphragm dysfunction. The interventional procedures were diaphragm plication in three patients (0.8%) and tracheotomy in three patients (0.8%). Three of these patients had a single ventricle and three had biventricular physiology. The median time after surgery for these procedures was 36 days. One patient (0.2%) died in the intensive care unit. The mean length of stay in the intensive care unit and hospital was 36±12 and 48±21 days, respectively. CONCLUSIONS Diaphragm dysfunction should be kept in mind in patients undergoing congenital heart surgery and in those who need prolonged intubation during the postoperative period. Ultrasonography is a non-invasive diagnostic tool which can be used to identify diaphragm dysfunction and the best course of management of this clinical condition.