This study aimed to explore the pattern of lung ultrasound (LUS) in newborns with congenital heart disease (CHD) and to investigate the accuracy of LUS assessing pulmonary overflow (PO) during the first days of life. Lung ultrasound was performed in 51 newborns during the first days of life, and newborns were classified in two groups depending on the predisposition to develop POas evaluated by the abundance of B-lines. The results were compared to the physical examination (PE), chest X-ray, and echocardiography. In both groups there were no differences in abundance of B-lines during the first days of life, but those with a type of CHD with a trend to develop PO had a higher B-lines score after 72 h (p < 0.05) with a good correlation with echocardiography findings and with a better sensibility than PE and chest X-ray. We found that LUS is a reliable tool for the diagnosis of PO and may be useful to monitor and optimize therapy, which should be further validated in multicentre studies.