Alteration of the immune system is one of the major mechanisms responsible for complications in severe acute pancreatitis (AP). The aim of our study was to provide a complex evaluation of peripheral blood monocyte subsets, natural killer cells (NK cells) and cytotoxic T lymphocytes in patients with different severity forms of AP.20 patients with mild AP and 15 with severe AP (S-AP) were included in our study. Peripheral blood mononuclear cells were studied on days 1-3, 5, 10 and 30, by means of flow cytometry.In peripheral blood of patients with pancreatitis, we found a marked increase in total monocyte count. In S-AP, circulating monocytes were significantly activated, which was presumed from increased expression of HLA-DR, CD54, CD69 and CD25. Concurrent increased expression of CD95 (FasR) may indicate enhanced susceptibility of these cells to apoptosis. In patients with S-AP, a dramatic depletion of circulating NK cells (CD16/56 and CD3- CD8+) was found along with a reduction of circulating CD3+ CD8+ lymphocytes (cytotoxic T lymphocytes).Our findings suggest profound disturbances of innate cellular immunity in patients with S-AP.