Cancers of an unknown primary site are heterogenous with respect to their clinical and pathologic features. They are generally very aggressive, but specific favorable subsets have a better prognosis. For these favorable subsets, taxane based chemotherapy is very effective for a subset of woman with papillary serous peritoneal adenocarcinoma. A 52 year-old woman underwent [(18)F]-FDG PET/CT for routine health screening. On PET/CT, multiple hypermetabolic lymph nodes were detected in the paraaortic spaces, and there were no other hypermetabolic abnormalities. The patient was diagnosed with an unknown primary cancer that probably originated from the ovary or peritoneum, according to clinical studies and biopsy results. This was not a typical case of a favorable subset of cancer of an unknown primary site, but the tumor showed complete biologic response to taxane based chemotherapy as revealed by PET/CT, and necrotic tumor cells were confirmed by surgery.