A lung nodule, also known as a pulmonary nodule, is an abnormal growth that forms in a lung and appears as a shadow or spot on the lung on X-rays or scans. Most lung nodules are benign, meaning they are not cancerous, and do not require treatment. However, some nodules can be cancerous, and all nodules should be carefully evaluated because lung cancer is the leading cause of cancer deaths in U.S. adults. Lung nodules are often discovered incidentally during imaging for other health conditions, such as a chest X-ray or CT scan. The most common causes of lung nodules are inflamed tissue due to an infection or inflammation or benign lung tumors. If a lung nodule is suspected to be cancerous, tests such as a biopsy or surgery may be needed to confirm the diagnosis and determine the best course of treatment. Symptoms of lung nodules are usually absent, but if present, they may include coughing, coughing up blood, wheezing, shortness of breath, and respiratory infection.