A hidden primary tumor presenting as an isolated lung mass is a diagnostic challenge to physicians because the diagnosis of lung cancer is likely to be made if the histologic findings are not inconsistent with lung cancer. A large lung mass was found incidentally in a 59-year-old man. Although adenocarcinoma was diagnosed by percutaneous needle biopsy, thyroid transcription factor-1 (TTF-1) immunostaining was negative, raising suspicion that there was another primary site. There was no abnormal finding except for the lung mass on a $^{18}FDG$-PET/CT scan and the patient did not complain of any discomfort. Finally, prostatic cancer was confirmed through the study of tumor markers and prostate-specific antigen (PSA) immunostaining. Because of the rare presentation of a single lung mass in malignancies that have another primary site, physicians should carefully review all data before making a final diagnosis of lung cancer.