Primary lung cancer in dogs — as distinct from cancer that has spread (metastasised) to the lungs from elsewhere — arises from the cells lining the airways or air sacs. Adenocarcinoma is the most common type. The tumour typically presents as a solitary mass within one lung lobe, often discovered incidentally on chest X-rays taken for other reasons. When symptoms are present, they may include coughing, exercise intolerance, or laboured breathing. The outlook depends largely on whether the tumour is a single mass (better prognosis) or multiple nodules (worse), whether lymph nodes are involved, and whether the tumour can be completely removed by surgery. Dogs with a single lung mass and no lymph node involvement have the best outcome following lung lobectomy, with some living two or more years. Primary lung cancer represents only about 1% of all canine tumours and is not associated with strong breed predispositions.