Procalcitonin (PCT), the precursor protein of calcitonin, has proved to be a good marker of "acute phase" response in bacterial infections, particularly in severe and generalized one. The diagnostic utility of PCT in community-acquired respiratory tract infections is discussed in the article, particularly with regard to community-acquired pneumonia and the utilization of PCT as a monitoring and prognostic tool in the course of disease. Our review of the available literature on the subject enables us to state the following: (1) Serum PCT levels increased significantly only in bacterial respiratory tract infections. (2) Increases in serum PCT concentrations were related to the intensity of systemic inflammatory response in the course of disease; very high PCT levels were associated with poor prognosis. (3) Measurements of PCT concentrations were useful in clinical decision making as to antimicrobial treatment. (4) Testing the serum PCT level not only supplements the range of available markers of "acute phase" response, but also provides some additional information about ongoing systemic inflammation.