BACKGROUND : Cancers of unknown primary origin (CUPs) are reported to be the 3-4th most common causes of cancer death. Recent years have seen advances in mutational analysis and genomics profiling. These advances could improve accuracy of diagnosis of CUPs and might improve the prognosis of patients with CUPs. METHODS : A 76-year old male with an adenocarcinoma of unknown primary origin in the lung presented with another tumor of the palate mucosa. The tumor cells in the pleural effusion were all negative for immunohistochemical markers (TTF-1 and Napsin A) and lung-specific oncogenic driver alterations (EGFR mutation and ALK translocation). The tumor of the palate mucosa was likewise identified as an adenocarcinoma, and the cells showed cytological similarities with the tumor cells in the pleural effusion; TTF-1, Napsin A, EGFR mutation and ALK translocation were all negative. This result suggested that origins of the tumors of the palate mucosa and in the lung were the same, even though the origin had not yet been determined. Next, we addressed whether the tumor of the palate mucosa was a primary tumor or not. Secretory carcinoma (SC), which is a common type of minor salivary gland tumor (MSGT), was suspected; however, mammaglobin was negative and ETV6-NTRK3 (EN) fusion was not observed. Other MSGTs were excluded based on histological and immunohistochemical findings. Furthermore, an additional examination demonstrated an oncogenic KRAS mutation at codon 12 (p.G12D) in both palate tumor and in pleural effusion. KRAS mutation is known to exist in one-third of lung adenocarcinomas (LUADs), but quite rare in MSGTs. The possibility of metastasis from other organs was considered unlikely from the results of endoscopic and imaging studies. This result indicated that the primary site of the CUP was indeed the lung, and that the tumor of the palate mucosa was a metastasis of the LUAD. CONCLUSIONS : A tumor of the palate mucosa that showed diagnostic difficulties was determined to be a metastatic LUAD by genomic alterations and histopathological findings.