Interleukin 18 (IL-18) is a new mediator and modulator of the immune response; its role in acute pancreatitis (AP), however, has not yet been fully explained. The aim of our study was to evaluate the profile IL-18 serum concentrations in the course of acute pancreatitis. The prospective study involves 30 patients with AP ( n = 15 with mild AP and n = 15 with severe AP) as well as 10 healthy subjects. AP severity was defined according to Ranson's and Balthazar's criteria, supplemented by serum CRP concentration measurements. In the course of hospitalization, 2 patients with severe AP died. Serum IL-18 and plasma polymorphonuclear leukocyte elastase (PMN-E) concentrations were measured at admission (day 1) and on days 2, 3, 5 and 10. In both the mild and the severe forms of AP, serum IL-18 concentration was significantly higher than in the healthy controls. In severe AP, serum IL-18 reached the highest levels in all observed periods compared to that in patients with mild AP. Significant correlations, calculated for day 1, were found between serum IL-18 and plasma PMN-E (Rs = 0.514, P < 0.001) and between IL-18 and CRP (Rs = 0.463, P < 0.001) levels. Serum profile IL-18 during AP indicates that this cytokine was released early after AP onset and may play the key role in inflammatory and immune response. Positive correlation between serum IL-18 and commonly known early prognostic markers of AP severity suggest that serum IL-18 concentrations may represent another early marker indicating severe course of AP.