Procalcitonin (PCT) is a propeptide of calcitonin which is normally produced in the C-cells of the thyroid glands. Although physiologic levels of PCT remain very low, a dramatic increase in serum PCT levels is observed during severe systemic infections. It is not infection per se but infection accompanied by severe systemic reactions or poor organ perfusion that increases procalcitonin levels. These properties make procalcitonin less useful for the diagnosis of simple infections but a very promising marker of severe infections especially in the critical care setting.