The serum concentration of procalcitonin (PCT) is specifically elevated in severe bacterial infections. In Japan, PCT has been used as a serum marker for bacterial sepsis since February, 2006. However, the evidence of it in respiratory infectious diseases is limited. In the present study, we analyzed 57 episodes of systemic inflammatory response syndrome in 53 inpatients to investigate the usefulness of serum PCT measurement in respiratory infectious diseases. Although the sensitivity and a negative predictive value in common bacterial infections were low, the specificity and positive predictive value were 95% and 93%, respectively. This suggests that the significance of serum PCT measurement relies on confirming the diagnosis of common bacterial infections. Analysis using a receiver operating characteristic (ROC) curve demonstrated that serum PCT was a more reliable measure of bacterial sepsis than leukocyte counts in the peripheral blood or serum C-reactive protein. In cases of community-acquired pneumonia, except atypical pneumonia, the hospital mortality rate was significantly higher in PCT-positive patients than in PCT-negative patients (38% vs. 0%), indicating that serum PCT could be a factor predicting in-hospital death.