This report describes a case of prostate adenocarcinoma presenting with supraclavicular adenopathy and deep venous thrombosis in the ipsilateral arm. Biopsy revealed the enlarged node to be an undifferentiated adenocarcinoma of unknown origin, while CT scan evidenced widespread adenopathies. Because of the increase in serum PSA, the immunohistochemical staining of the biopsy specimen was reviewed and strong positivity for PSA suggested a prostatic origin. We emphasize the importance of PSA immunohistochemistry and serum PSA level monitoring in men presenting with carcinoma of undetermined origin and generalized lymphadenopathies.