Prostate cancer is rarely detected from abnormal chest radiographs. We report two cases of prostate cancer detected from pleural effusion. Case 1 is a 76-year-old man who consulted the department of internal medicine of our hospital with dyspnea and abdominal fullness. Pleural effusion and multiple hepatic tumors of unknown origin were pointed out, but he refused any further investigation or treatment for them. Six months later, he consulted a family doctor with urinary frequency and lumbago. Increased serum prostate specific antigen (PSA) level to 864 ng/ml was recognized, then he was referred to our department. Under diagnosis of prostate cancer, T4NOM1c, maximal androgen blockade (MAB) was performed. Serum PSA level was decreased once to 8.1 ng/ml, but then rose gradually and he died 13 months after the beginning of the therapy. Case 2 was a 78-year-old man who was referred to our department to determine the origin of carcinomatous pleuritis detected in a routine general check up of hepatitis C. The serum PSA level was increased to 12,900 ng/ml, and the diagnosis was prostate cancer, T3aNOM1c. Although MAB was performed, the serum PSA level did not decrease markedly. He died 16 month after the beginning of the therapy.