The procalcitonin (PCT) is a sensitive and reliable biochemical marker used in diagnosing and monitoring of bacterial infections. The procalcitonin assay allows for effective evaluation of the patient's health status. Elevated PCT serum level is correlated with progression and generalization of the infection. In severe infections, such as sepsis or septic shock, the PCT may reach concentrations even up to 1000 ng/ml (with a normal range below 0.1 ng/ml). High PCT levels in the first hours of the developing infection advocates for the bacterial etiology. Therefore, procalcitonin is used in differential diagnosis of severe bacterial and viral infections. Rapid and accurate diagnosis facilitates introduction of the effective therapy. Rapid decrease in PCT level after applying the therapy confirms its efficacy (control assay may be performed after 24 hours since the therapy has been applied). The usefulness of the PCT is supported by the fact, that in pathologic conditions, such as trauma (including surgery procedures), viral infections or autoimmunologic diseases, the PCT level is only slightly increased or remains in a normal range. The PCT level assay enables the detection of the developing infection already in the latent stadium, before characteristic clinical symptoms appear. Therefore, it is very important to monitor the PCT serum level. Further research will allow to determine the accurate diagnostic value and the clinical application of the PCT level as a marker of infection.