A person with small cell carcinoma of the lung has abnormal cells in the lung that multiply out of control. These cells can form tumors and spread to other parts of the body.

Small cell carcinoma is also called oat cell cancer, because under the microscope, the cells look like oats. Oat cells are small, but they can multiply fast and create large tumors.

Small cell carcinomas may produce hormones, such as antidiuretic hormone and adrenocorticotropic hormone. For this reason, patients with small cell carcinoma can have tumors that release extra hormones into the bloodstream. The production of extra hormones causes symptoms, and is called paraneoplastic syndrome.

Small cell carcinomas make up about 20% of lung cancers. Small cell carcinomas are associated with smoking and radiation exposure.