An 84-year-old man was referred to our hospital with swollen right cervical lymph nodes. Computed tomography showed right supraclavicular and mediastinal lymph node enlargement, and fluorodeoxyglucose positronemissiontomography showed multiple areas of abnormally increased radioactivity inthe right supraclavicular and mediastinal lymph nodes, right ninth rib, and left fifth and seventh ribs. Biopsy of the right supraclavicular lymph node revealed metastatic adenocarcinoma with partial immunohistochemical staining for prostate specific antigen (PSA). Serum PSA levels were not elevated (2.01 ng/ml). An 8-core transrectal prostatic biopsy was negative. Thus, we could not determine the primary site of the adenocarcinoma. The patient was diagnosed with carcinoma of an unknown primary site and followed without chemotherapy. Four years later, he was referred to our hospital due to right hydronephrosis. Serum PSA level was 31. 1 ng/ml. The tumor was not palpable by rectal examination. A 12-core transrectal prostatic biopsy revealed a poorly differentiated adenocarcinoma. Computed tomography revealed metastases in the left axilla, para-aortic, and pelvic lymph nodes as well as in the lung. We diagnosed the patient with prostate cancer, and combined androgen blockade (CAB) was administered. Metastases in the lymph nodes, lung, and bone were reduced on imaging after 1 month of therapy. Therefore, a definitive diagnosis of prostate cancer T1cN1M1c was made. Dutasteride had been administered as a benign prostate hyperplasia treatment 2 years before his first visit, which may have made the definitive diagnosis of prostate cancer difficult. In contrast, dutasteride may have delayed the progressionof prostate cancer inthis patient.