Abstract: Objective To study the role of serum procalcitonin (PCT) in clinical diagnosis of severe pneumonia. Methods Serum PCT level, and C-reactive protein level, number of white blood cells, percentage of neutrophils in 46 severe pneumonia patients, 40 mild pneumonia patients and 50 controls admitted to our hospital from November 2009 to September 2011 were analyzed. Results The serum PCT level was 17.34±12.32 ng/ml in severe pneumonia patients, which was significantly higher than that in mild pneumonia patients (9.34±3.34) ng/ml and controls (0.23±0.06) ng/ml (P< 0.05). The serum PCT level was 19.34±8.85 ng/ml in bacterial pneumonia patients, which was significantly higher than that in viral pneumonia patients (6.43±0.32) ng/ml, mycoplasma pneumonia patients (5.34±0.16) ng/ml and controls (P< 0.01). Although the C-reaction protein level, number of white blood cells, percentage of neutrophils were higher in pneumonia patients, their sensitivity and specificity were lower than those of PCT. The serum PCT level was not influenced by non-infectious factors and could reflect the severity of pneumonia, and thus could be used as an effective indicator to distinguish bacterial pneumonia from viral pneumonia or mycoplasma pneumonia. Conclusion Serum PCT plays an important role in clinical diagnosis of severe pneumonia and in assessment of its severity.