Procalcitonin (PCT) is a protein synthesised by the thyroid C-cells, inside which is cut into calcitonin (CT) and catacalcin. It remains undetectable in serum in normal conditions. Its level increases during inflammation and in small-cell lung cancer. There have been studies suggesting that the PCT level increases in medullary thyroid carcinoma (MTC). So far there have been no reports that would assess the usefulness of PCT detection in MTC. Our aim was to evaluate the usefulness of serum PCT assays in the patients with MTC. We investigated 24 patients at the age of 17-78 years, all after a total thyroidectomy due to MTC. All patients had serum CT concentrations measured with the radioimmune assay (RIA-DPC). The upper limit of CT level was 60 pg/ml. The serum PCT was evaluated with immunochromatographic kit . The reaction was considered positive when PCT level exceeded 0.5 ng/ml. In all cases the C-reactive protein serum level was measured. The statistical analysis was performed with the Statistica 5.1G. The CT levels in all patients varied from 0 to 1410, mean 603.8 pg/ml. In 8 patients the CT level was within normal range, in 6 patients it was marginally and in 10 patients markedly elevated. The PCT test was considered positive in 16 patients. There was correlation between serum PCT and CT concentrations (Spearman test, p<0.0001). The PCT levels varied considerably between patients with normal, marginally and markedly elevated CT levels (Kruskal-Wallis test, p=0.0013). All patients had normal CRP values. Fisher?s exact test revealed a correlation between serum PCT and CT increase (p=0.04). Further studies of a bigger group of patients should be considered, so far, the PCT assay can be thought useful in cases of unclear CT concentration.