Background: Sepsis is a leading cause of mortality in critically ill patients. However, differentiating sepsis from non-infectious triggers of the systemic inflammatory response syndrome (SIRS) is difficult. Procalcitonin is useful biomarker of systemic inflammatory response to infection. Its level rises in response to a proinflammatory stimulus, especially of bacterial origin and not rise significantly with viral or non infectious inflammation. Objectives: The aim of this study was to determine the relation of procalcitonin (PCT) level with the development of organ failure and mortality in critically ill septic ICU patients Patients and Methods: The current study was conducted on 60critically ill adult septic patients aged between 18-60 years old of both sex with anticipated stay of >48 hours. All patients were assessed clinically with haemodynamic and full laboratory monitoring, CRP, a PCT level and SOFA score were calculated in the1^st and the 4^th day of admission.Results: There was significant increase in PCT level between three groups and no significant difference between groups as regard CRP. According to SOFA score there was significant difference between three groups. There was a positive correlation between PCT level and SOFA Score (r = 0.924, P = 0.001) while there was no correlation between CRP and SOFA score (r = -0.233, P = 0.091). Conclusion: PCT is a good diagnostic and prognostic marker of sepsis. PCT shows a closer correlation than that of CRP with the severity of infection and organ dysfunction.