Lung cancer, also known as lung carcinoma, is a malignant tumor that begins in the lung. Lung cancers are caused by genetic damage to the DNA of cells in the airways, often exacerbated by cigarette smoking, or inhaling damaging chemicals. Damaged airway cells sometimes gain the ability to proliferate unchecked, causing the growth of a tumor. Without treatment, lung tumors can spread throughout the lung, damaging lung function. Eventually lung tumors metastasize, spreading to distant parts of the body, and causing varying disease. Lung cancers are classified based on the cells they originate from. Around 15% are small-cell lung cancers, while the remaining 85% (the non-small-cell lung cancers) are adenocarcinomas, squamous-cell carcinomas, and large-cell carcinomas.

Early lung cancer often has no noticeable symptoms, and can only be detected by lung cancer screening programs. As the cancer progresses, most people experience symptoms of general respiratory problems: coughing, shortness of breath, and/or chest pain. These can be accompanied by a wide variety of symptoms depending on the location and size of the tumor. Many develop symptoms due to metastases – most commonly to the brain, bones, liver, and adrenal glands. Some tumors cause the release of various hormones that impact body-wide functions, causing an array of symptoms collectively called paraneoplastic syndromes. Those suspected of having lung cancer typically undergo various imaging tests to determine the location and extent of any tumors. Definitive diagnosis of lung cancer requires a biopsy of the suspected tumor be examined by a histologist under a microscope. After diagnosis, lung cancer is staged based on how far it has spread. Cancers caught at an earlier stage tend to have better prognoses.

Treatment for early stage lung cancers tends to include surgery to remove the tumor, sometimes followed by radiation therapy and chemotherapy to kill remaining cancer cells. Later stage cancers are treated with radiation therapy and chemotherapy alongside targeted molecular therapies and immune checkpoint inhibitors. Even with treatment, lung cancer is often deadly, with around 19% of people surviving five years from their lung cancer diagnosis. Survival is higher in those diagnosed at an earlier stage, diagnosed at a younger age, and in women compared to men.

The vast majority (85%) of cases of lung cancer are due to long-term tobacco smoking. About 10–15% of cases occur in people who have never smoked. These cases are often caused by a combination of genetic factors and exposure to radon gas, asbestos, second-hand smoke, or other forms of air pollution. Lung cancer may be seen on chest radiographs and computed tomography (CT) scans. The diagnosis is confirmed by biopsy, which is usually performed by bronchoscopy or CT-guidance.

The major method of prevention is the avoidance of risk factors, including smoking and air pollution. Treatment and long-term outcomes depend on the type of cancer, the stage (degree of spread), and the person's overall health. Most cases are not curable. Common treatments include surgery, chemotherapy, and radiotherapy. NSCLC is sometimes treated with surgery, whereas SCLC usually responds better to chemotherapy and radiotherapy.

Worldwide in 2020, lung cancer occurred in 2.2 million people and resulted in 1.8 million deaths. It is the most common cause of cancer-related death in both men and women. The average age at diagnosis is 71 years. In most countries the five-year survival rate is around 10 to 20%, although outcomes typically are worse in the developing world.