Bronchogenic carcinoma, or lung cancer, is the leading cause of cancer deaths in the United States and most of the developed world. In the 1990s, more women were dying of lung cancer than of breast cancer, which historically had been the major cancer killer in women. Although its cause has been traced to many environmental factors, such as asbestos, benzene, industrial chemicals, and radon exposure, most lung cancers (80-90 percent) are associated with cigarette smoking. There are two types of lung cancers: non-small cell carcinoma and small cell carcinoma, or oat-cell carcinoma. Of the two, the oat-cell carcinoma is the most aggressive and accounts for about 20-25% of all cases. There are three types of tumors that characterize non-small cell lung cancers: squamous-cell carcinomas, adenocarcinomas, and large-cell carcinomas.