Undifferentiated lung cancer is a general term for a type of lung cancer that is poorly differentiated. Undifferentiated carcinoma has high malignancy, rapid growth, early occurrence of extensive metastasis in lymphatic and blood channels, and is more sensitive to radiotherapy and chemotherapy, but has the worst prognosis. Undifferentiated small cell carcinoma and undifferentiated large cell carcinoma can be divided into two types according to their histological patterns. Undifferentiated small cell carcinoma is often called small cell lung cancer, accounting for about 20% of all lung cancers. It occurs in the large bronchial tubes near the bronchi and is a central type of lung cancer, and mostly forms giant masses, with brittle cancer tissue. The cancer cells are fast growing, invasive, rapid in development, metastasize through lymphatic and blood channels in early stage, and often metastasize to lung, liver, bone marrow, bone and adrenal gland. Small cell lung cancer responds well to chemotherapy and is sensitive to radiotherapy, but the prognosis is extremely poor. Undifferentiated large cell carcinoma is often referred to as large cell lung cancer. Because of its large cell size and diverse morphology, it can be subdivided into six subtypes, including large cell neuroendocrine carcinoma, compound large cell neuroendocrine carcinoma, basal-like carcinoma, lymphoepithelioid carcinoma, clear cell carcinoma, and large cell carcinoma with transverse myeloid phenotype. Large cell undifferentiated carcinoma accounts for 9% of all lung cancers. The surgical resection rate is higher than that of small cell lung cancer, and the general five-year survival rate is higher than that of small cell lung cancer and lower than that of the better differentiated lung adenocarcinoma and squamous carcinoma.