Searching for the primitive tumor of a peritoneal carcinomatosis is sometimes a difficult task even with modern diagnostic modalities. However, it is of paramount importance to identify primary tumors in patients with peritoneal carcinom.atosis, which are responsive to systemic chemotherapy and/or locoregional treatment. We presented a 74- years old female patient with massive malignant ascites. Abdominal computed tomography (CT) and Gallium scan failed to detect primary origin of peritoneal carcinomatosis. F-18 fluorodeoxyglucose (FDG) PET/CT showed a FDG-avid lesion in right ovary with multiple FDG-avid nodules over mesentery and retroperitoneum. Under the impression of right ovarian cancer with peritoneal seeding, she underwent cytoreductive surgery with total hysterectomy, bilateral salpingo-oophorectomy, bilateral pelvic lymph node, para-aortic lymph node dissection and partial omentectomy. Chemotherapy with avstin, taxol and carboplatin was administered following operation. The patient remained well after 4-month of follow-up. Her CA-125 and CA-199 went down and ascites resolved completely. Our data strongly suggest the combined F-18 FDG PET/CT is helpful in identifying primary tumor origin in some patients with peritoneal carcinomatosis, whose primary cancer cannot be detected by conventional diagnostic modalities.