Background and study aim: Spontaneous bacterial peritonitis is a serious condition that needs rapid diagnosis and rapid management due to its serious sequelae. SBP is diagnosed when the polymorpho-nuclear cells count in the ascetic fluid exceeds 250 cell/μL. Mean platelet volume (MPV) was found to be significantly larger in the cirrhotic patients with ascetic fluid infection than cirrhotic patients without ascetic fluid infection. In our study we aimed to assess the role and clinical performance of MPV as a diagnostic marker of SBP. Patients and Methods: This cross sectional study was performed on 124 cirrhotic patients with ascites. They were classified into two groups according to ascetic fluid PMN count into two groups. Group I: 38 patients with ascetic fluid infection, PMN >250 cell/μL and group II: 86 patients without ascetic fluid infection, PMN count <250 cells/μL. Results: The MPV was significantly higher among patients with ascetic fluid infection (11.1±1.2 vs 9.4±1.1 p<0.001). Blotting the ROC curve, MPV was proved to diagnose SBP at a cut off value of 10.45 fL with sensitivity and specificity of 74.4% and 78.9% respectively. Conclusion: MPV is a useful diagnostic marker that can predict the presence of SBP in cirrhotic patients with ascites.