Carcinomas of unknown primary are cancers that are pathologically diagnosed as carcinomas based on the findings of metastatic lesion biopsy but in which the primary site remains unknown even after thorough and detailed examinations.In the clinical setting, pathological diagnosis and diagnostic imaging are very important for cases of carcinoma of unknown primary. When examinations do not identify the primary tumor, carcinoma cases are treated as carcinoma of unknown primary.In clinical practice, it is very important not to miss cases of favorable prognosis group, which is 20% of unknown cancer. Recently, microarray studies have been performed for cases of carcinoma of unknown primary.