Cardiopulmonary bypass (CPB) induces neutrophil activation, degranulation, and a systemic inflammatory response. Matrix metalloproteinase (MMP)-9 exists in neutrophils and is released on neutrophil activation. Increased levels of MMP-9 have been observed in patients undergoing CPB. We designed the present study to determine whether MMP-9 is derived from neutrophils during CPB. Twenty-one patients undergoing elective coronary artery bypass grafting with or without CPB were included in this study. Blood was collected and analyzed for MMP-9 and tissue inhibitor of metalloproteinase (TIMP)-1. Neutrophils were also isolated and examined for MMP-9 production and mRNA expression. Plasma levels and activity of MMP-9 increased significantly 2-6 h after beginning CPB, whereas the MMP-9 levels in patients with off-pump cardiac surgery did not increase. The neutrophil content of MMP-9 and mRNA increased significantly 2 h after beginning CPB. The plasma levels of TIMP-1 increased gradually for 6 h, whereas the MMP-9/TIMP-1 ratios were increased 2-4 h after beginning CPB. The present study demonstrated that CPB causes an increase in the concentration and activity of plasma MMP-9. The corresponding increase in neutrophil MMP-9 expression and production suggests that MMP-9 is derived primarily from neutrophils and may contribute to the inflammatory response associated with CPB.