A 56-year-old man presented to our hospital with dyspnea on exertion for two months. Bilateral pleural effusions were found, and a close examination revealed a chylothorax, including adenocarcinoma. The primary tumor could not be identified by systemic examination. Therefore, the patient was diagnosed with cancer of unknown primary (CUP) presenting with chylothorax. Chemotherapy was administered for CUP, and thoracentesis, pleurodesis, ascites puncture, and nutritional therapy were performed for chylothorax and chylous ascites. Although drainage frequency and tumor marker levels (CA19-9, DUPAN-2, and Span-1) temporarily decreased, disease control deteriorated, and the patient died 12 months after the initial diagnosis.