Abstract: The aim of this study is evaluation of procalcitonin role in the diagnosis of infectious and non infectious inflammation. This cross-sectional study was conducted in one hundred patients in Baqiyatallah Hospital of Iran in 2008. Patients suspected to infection were recruited to study. They were divided to four groups as: systemic inflammatory response syndrome, sepsis, sepsis syndrome and septic shock. Procalcitonin quantitative was assayed by immunoluminometric kit manufactured in Germany. Procalcitonin level was divided to four groups in <0.5 ng mL-1 compatible for SIRS, 0.5-2 ng mL-1 for sepsis and 2-10 ng mL-1 for sepsis syndrome and >10 ng mL-1 for septic shock. Data was analyzed by SPSS 13 for window software; T student test, ANOVA and Chi-square were used. In this study 53(53%) of subjects were men with mean age of 56.16 ± 19.5 years old. The diagnosis was SIRS in 36%, sepsis in 38%, sepsis syndrome in 14% and septic shock in 12% of cases. Procalcitonin level was less than 0.5 ng mL-1 in 61% and more than 10 ng mL-1 in 10% of patients. Procalcitonin level showed significant association with septic shock, positive blood culture and mental dysfunction. Ultimately this study showed that high level of procalcitonin can differentiate septic shock from SIRS and other stages of infection. Dysfunction of mental status and high level of procalcitonin can determine septic shock.