In febrile children and adults, it is frequently difficult, based on the sole clinical examination, to differentiate a bacterial illness from systemic inflammatory syndromes or severe viral infections. However, the positive and rapid diagnosis of a severe bacterial infection or a sepsis is essential to initiate lifesaving therapies. Among the numerous infectious biomarkers that have recently been investigated, procalcitonin has the best diagnostic yield. Plasma levels below 0.5 microg/l usually rule out a severe bacterial disease, whereas values above 2 microg/l are strongly indicative of a bacterial sepsis. The usefulness and the limitations of the measurement of procalcitonin as a diagnostic and a prognostic tool during severe bacterial infections are discussed in this paper.