Background: Spontaneous bacterial peritonitis (SBP) is the most common infection with a high morbidity and mortality in cirrhotic patients with ascites. Mean platelet volume (MPV) is an indicator of active platelets with more proinflammatory actions. Aim: The present study was aiming to prove that increased MPV is an indicator of inflammatory processes in SBP. It was conducted on 50 patients with liver cirrhosis and ascites and 30 healthy subjects as controls. Routine laboratory investigations including: CBC with MPV estimation, ESR, CRP, ascitic fluid examination for cirrhotic patients and abdominal ultrasound done for all patients and compared to controls. Results: MPV was significantly higher in patient than controls (11.0±1.2 and 9.9±0.9) respectively with (P.value < 0.001). At a cutoff value ≥ 10.8fl, MPV had 82.5% sensitivity and 100% specificity for detecting SBP. Conclusion: MPV increases in cirrhotic patients with ascitic fluid infection, indicating occurrence of systemic inflammatory response.