We sought to study Toll-like receptor 4 (TLR4) expression on peripheral blood mononuclear cells (PBMCs) during the early stage of human acute pancreatitis (AP). Thirty consecutive patients with acute pancreatitis admitted within 24 hr of onset of abdominal pain were enrolled prospectively in this study. Blood samples were taken by venipuncture at admission and on the third and seventh days after admission. PBMCs were isolated, and TLR4 and CD14 expression on PBMCs was detected by flow cytometer. Serum tumor necrosis factor (TNF)-alpha, interleukin, and lipase were detected simultaneously. Relations among these parameters were analysis. In mild AP, TLR4 expression increased on the first day of admission and then continued to decline for several days. On the seventh day, TLR4 expression was almost normal compared with that of the normal control. The alteration of serum TNF-alpha was coincidence with TLR4. We conclude that mononuclear-macrophages might be ignited through TLR4 (the gatekeeper of the innate immune system) and lead to production of TNF-alpha.