Background: Spontaneous bacterial peritonitis (SBP) is characterized by the presence of infected ascetic fluid without any previous history of intra-abdominal source of infection. In addition to the presence of increased level of polymorph nuclear leukocytes exceeding 250/μL in the ascetic fluid. Objective: to evaluate the role of mean platelet volume (MPV) as diagnostic marker of SBP as well as a prognostic factor to follow up the response to treatment of SBP. Methodology: 40 patients suffering from cirrhosis were included in the study. All of them were divided into 2 groups, SBP and non-SBP, each consisted of 20 patients. Both groups were subjected to full clinical assessment, laboratory test evaluation, microbiological culture and sensitivity, abdominal ultrasonography and calculation of Child Pugh score. Patients with SBP were treated and followed up after five days. Results: We found that there was a statistical significant increase in the MPV levels in cirrhotic patients with SBP compared to cirrhotic patients without SBP (p < 0.001). Also, the statistical increase was observed in the SBP group with respect to MPV, ESR and C-reactive protein (CRP). ROC curve analysis suggested that the optimum MPV level cut-off point for cirrhotic patients with SBP was 8.4 fl, with a sensitivity, specificity, negative predictive value (NPV), and positive predictive value (PPV) of 73%, 85.7%, 75.7% and 83.9%, respectively (area under curve: 0.84), follow up patients with SBP after end of treatment showed statistically significant difference in MPV measured before 8.5±0.6 fl and after treatment 8.1±0.8 fl (P<0.0001). Conclusion: MPV measurement can be used as a valuable diagnostic tool of SBP in cirrhotic patients as well as a prognostic marker to follow up response to treatment because it is rapid, easily applicable and valuable method