Peritonitis is an inflammatory process which involves the peritoneal cavity and its serous surfaces (the peritoneum). The purpose of this study was to analyze the peritoneal fluid in dairy cows suffering from peritonitis. In the control case study, 45 dairy cows were selected from the dairy farms in Tehran Province. Eighteen cows that showed no sign of peritonitis were considered for the control group (C) and the remaining 27 were divided into two groups: the group with peritonitis (A, n=21) and the group suffering from some miscellaneous disorders (B, n=6) in abdominal cavity (displacement of the abomasum, ascites, as well as liver and spleen abscesses). In the peritoneal fluid analysis, only one case in group (A) had traditional indicators of peritonitis and, in the remaining cows, other measures of inflammation (changes in cellular composition) were observed. Statistical analysis was performed using Chi-square for qualitative data and Kruskal-Wallis, ANOVA, and ROC curve for the quantitative ones. Finally, it was found that peritoneal fluid analysis does not have the required efficiency for the diagnosis of peritonitis in many cases and to reach the diagnosis with high accuracy and validity in bovine peritonitis, simultaneous use of laparoscopy and abdominocentesis is recommended.