Previous studies have shown that matrix metalloproteinase 9 (MMP-9) degrades basement membrane components in inflammation, but the change of serum MMP-9 level in the progression of acute pancreatitis remains unclear. The aim of our study was to assess the value of MMP-9 as a prognostic marker in acute pancreatitis. The prospective study included 10 patients with severe acute pancreatitis (SAP) and 10 patients with mild acute pancreatitis. The study also enrolled 10 healthy individuals as control. The serum MMP-9 level, serum C-reactive protein (CRP) level, serum tumor necrosis factor alpha (TNF-alpha) level and acute physiology and chronic health evaluation (APACHE) II score were measured at 1 hr and 48 hrs after admission. APACHEII scores and serum MMP-9, TNF-alpha and CRP levels were significantly increased in patients with SAP compared to those with mild acute pancreatitis and control subjects at 1 hr after admission (p < 0.01). When the states of illness were improved, the levels of the above-mentioned markers were decreased in patients with SAP at 48 hrs after admission (1 hr vs 48 hrs, p < 0.01 or p < 0.05). Furthermore, significant positive correlation was found between serum MMP-9 level and serum TNF-alpha level, serum CRP level or APACHEII score in patients at 1 hr after admission (MMP-9/TNF-alpha, r = 0.956; MMP-9/CRP, r = 0.935; MMP-9/APACHE II score, r = 0.957; p < 0.01). These results suggest that MMP-9 is involved in the deterioration of SAP and serum MMP-9 level is a valuable assessment marker for the severity of SAP.