Objective : The "normal-sized ovary carcinoma syndrome (NOCS)" is defined as a clinical situation, in which diffuse metastatic malignant disease of the abdominal cavity of the female with normal-sized ovaries is noted, but no origin is assigned by preoperative or intraoperative evaluation.The aim of our study was to evaluate the value of 18F-FDG PET-CT in detecting the primary cause of NOCS.Materials and methods: This is a retrospective study of 17 consecutive women with NOCS, who had whole-body PET-CT and pathologic examination at our institution from 2005 to 2010.interpretation criteria 1:SUVmax>3.0 in ovary and any abnormal focal lesion of increased 8F-FDG uptake greater than the surrounding background activity of the organ being examined was interpreted as a possible site of malignant disease.High uptake of serous membrane was interpreted as metastasis.Interpretation criteria2: visible high uptake and visible abnormal thickening of and projections in the stomach wall was interpreted as a possible site of malignant disease.image interpretation of 18F-FDG PET-CT was based on the consensus of 2 nuclear medicine physicians.Results: The result of interpretation criteria 1: PET detected 13 occult primary tumors in 17 NOCS patients(detection rate,76.5%), including ovarian cancer(n=9), gastric cancer(n=2) with ovarian metastasis(krukenberg tumor), sigmoid colon carcinoma(n=1), peritoneal hemangiosarcoma(n=1), all 3 patients with false-negative PET findings was confirmed as ovarian cancer, and 1 patient with false-negative PET finding as gastric cancer.The positive predictive value was 100%.The result of interpretation criteria 2∶1 ovarian cancer with SUVmax2.5 of ovaries and was reported as malignant, and the gastric cancer without high uptake show abnormal thickening of the wall in CT, so, the detection rate is 88.2%.All 7 patients had tumor marker exam before PET show elevated serum CA125.Conclusions: PET/CT can be a valuable tool to identify a subset of patients with NOCS, ovarian cancer is main components of all the primary lesion with NOCS, gastric tumor containing mucus or signet ring cell carcinoma apt to invade ovaries to cause krukenberg tumor, PET-CT can detect primary gastric cancer and the ovarian metastasis at the same time.SUV is not fully for ovarian and gastric cancer, visual interpretation is very important.Elevated tumor marker can be indicator in NOCS.