Levels of C-reactive protein (CRP) and serum amyloid A protein (SAA) in blood are increased as acute phase proteins in patients with inflammatory conditions. Most of the currently used inflammatory markers, such as erythrocyte sedimentation rate and CRP or SAA levels, are non-specific parameters. By contrast, procalcitonin (PCT) has been reported to be selectively induced by severe infection in systemic inflammatory response syndrome (SIRS) and also in sepsis or multiorgan dysfunction syndrome. PCT expression is induced only slightly, if at all, by viral infections, autoimmune disorders, neoplastic disorders and trauma arising from surgical intervention. Serum PCT and SAA levels were compared in 93 patients with a CRP concentration higher than 100 mg/L and in 26 patients with a CRP concentration lower than 1.5 mg/L. In patients with high levels of CRP, all patients with sepsis and severe bacterial infection showed a significantly increased PCT concentration of more than 1.0 μg/L and it was possible to differentiate between the patients with neoplastic disorders and those with other inflammatory diseases. In patients with low levels of CRP, the PCT concentration was less than 0.3 μg/L and an increased PCT level was not seen in patients with autoimmune disorders or viral and fungal infections. These results suggest that determining the serum PCT level may be useful in the differential diagnosis of severe infection.