Procalcitonin (PCT), a 116 amino acid prohormon of calcitonin, is a novel diagnostic marker of severe bacterial infection. The aim of the study was to evaluate the diagnostic usefulness of serum procalcitonin concentration in severely sick patients admitted to the department of internal diseases. Fifty one patients were included into the study and divided into two groups: group A--36 severely sick patients with different medical problems without signs of bacterial infection; group B--15 patients with severe bacterial infection (sepsis--7 patients, pneumonia--6, pyelonephritis--1, peritonitis--1). Twenty eight healthy controls were also included into the study. Serum PCT concentration measured by immunoluminometric assay was undetectable or low in control group (range 0.0-0.1 ng/ml) and group A (range 0.0-1.8 ng/ml). In group B (range 0.0-183 ng/ml) markedly elevated PCT levels were observed in all patients with sepsis, peritonitis and some patients with pneumonia. We conclude that high serum PCT level support the diagnosis of severe bacterial infection.