The two case studies are describing 2 females with tumor-suspicious lung opacities including pleural effusions. Both patients have had a smoking history of more than 20 packyears and both underwent surgery for a breast respectively an ovarian cancer 10/15 years before. Conventional staining of the biopsies does not allow in most cases the differentiation between a primary lung carcinoma or the metastasis of an extrapulmonary tumor. Immunohistochemical techniques can give clues as to the origin of the primary tumor. The contribution of these methods in finding the correct diagnosis in the two presented cases is discussed. The cooperation between clinician and pathologist should be emphasized. The transfer of all important information is essential.