Carcinoembryonic antigen (CEA) is a tumor marker for detecting recurrences of adenocarcinomas such as colon cancer. In lung adenocarcinoma, CEA elevation can be found in both serum and malignant pleural effusion. However, CEA elevation in cytologically negative pleural effusion in the presence of adenocarcinoma without pleural infiltration has not been described. We here present the case of an 82-year-old man with incidental early stage adenocarcinoma of the right upper lobe showing CEA elevation in pleural fluid and serum despite negative cytological findings. Due to limited lung reserve the tumor was removed by wide wedge resection, but the visceral pleura was not affected and infiltration of the parietal pleura was ruled out by pleural biopsies. Serum and pleural CEA levels declined postoperatively as measured at 1 and 2 months follow-up. This case shows CEA elevation in serum and pleural fluid in early stage lung adenocarcinoma with negative cytology and no sign of pleural infiltration. Previous research revealed that CEA level in pleural effusion correlates to serum CEA and is significantly higher in adenocarcinoma of the lung than in other lung cancer entities. Firstly, this case suggests that determination of CEA levels can increase the diagnostic sensitivity in cases with cytologically negative pleural effusion suspicious of malignant origin and secondly, it contributes valuable information to the decision whether follow-up of pulmonary nodules or continuative diagnostics such as video-assisted thoracoscopic surgery (VATS) wedge resection is indicated.