Background: Peritoneal carcinomatosis (PC) is the most frequent and life-threatening types of metastasis in patients with gastric cancer (GC). Although the presence of tumor cells in peritoneal cavity is evaluated with microscopic observation by pathologists (Cy) or detection of mRNA of tumor specific molecules, no reliable method is available to quantify the accurate frequency of tumor cells. Methods: A total of 380 samples of acsites or peritoneal lavages were recovered from 243 patients with GC and 22 patients with liver cirrhosis (LC). Peritoneal fluids were obtained by laparotomy or paracentesis, and from subcutaneous intraperitoneal access port in case of the patients who received repeated intraperitoneal (IP) chemotherapy for PC. Cells were recovered by centrifugation of peritoneal fluids and immunostained with mAbs to CD45 and to CD326 (EpCAM). Using flowcytometry, the number of CD326(+)CD45(-) and CD45(+)CD326(-) cells, which were determined as tumor cells (T) and leukocytes (L), respectively, were calculated in 10⁴~10⁵acquired cells, and T/L ratio (TLR) was calculated. Results: Median (M) of TLR of the GC patients with PC(+) patients was 0.19% (0%-1868.44%, n=281), which was significantly higher than those of PC(-) patients (M=0%, 0%-0.30%, n=77, p<0.001) and LC (M=0%, 0%-0.028%, n=22, p<0.001). In PC(+) patients, 168 samples which was determined as positive cytology (Cy+) showed higher TLR as compared with 113 samples with Cy (-) (M=1.33%, 0%-1868.44% vs M=0%, 0%-1.13%, p<0.0001). In 37 patients who underwent repeated IP chemotherapy, TLR was markedly decreased after chemotherapy and the response was more sensitive than the changes in Cy or mRNA of CEA. Moreover, TLR of the patients with PC(+) GC before chemotherapy was significantly associated with their outcome. Median survival times (MST) of the patients whose initial TLR were <1.0%, 1.0%~10%, <10% were 765, 394, 271 days, respectively (p<0.001). Conclusions: TLR measured with flowcytometry well reflects the relative volume of living tumor cells in peritoneal cavity and thus could be a useful biomarker to predict the prognosis as well as the effectiveness of IP chemotherapy in patients with PC.