Backgronnd/Aims: The CA 125 is a glycoprotein expressed by coelomic epithelium during embryonic development. The elevation of CA 125 level was originally found in patients with epithelial ovarian cancer, and was observed in various benign and malignant diseases. The aim of this study was to determine the clinical significance of CA 125 in tuberculous peritonitis. Methods: The study was performed for 56 subjects with ascites. These subjects included patients with tuberculous peritonitis (22), liver cirrhosis (1 1), spontaneous bacterial peritonitis (11) and peritoneal carcinomatosis (12). CA 125 levels in serum and ascitic fluid were measured. The correlation between serum CA 125 and ascitic fluid ADA (adenasine deaminase) was investigated. Results: In all disease groups, the mean serum values of CA125 were higher than the cutoff level of 35 U/mL. However, there was no significant difference among groups (p>0.05). In tuberculous peritonitis, the ratio of CA125 1evel in ascitic fluid to CA125 level in serum was higher than that in liver cirrhosis and pertoneal carcinomatosis, but the difference was statistically insignificant. In tuberculous peritonitis, there was a positive correlation between CA 125 level in serum and ADA level in ascites (r=0.5889, p<0.01). Conclusions: It was concluded that possibility of tuberculous peritonitis must be considered in the diagnosis of patients with ascites and elevated CA 125. However, the elevated CA 125 is nonspecific finding in the diagnosis of tuberculous peritonitis and thus, has no special meaning in the differential diaginosis of ascites. (Kor J Gastroenterol1998;32:332-338)