To investigate the clinical significance of circulating hepatocyte growth factor (HGF) and the role of peripheral blood mononuclear cells (monocytes), which are a possible source of HGF, in patients with acute myocardial infarction. 37 patients with acute myocardial infarction and 13 normal control subjects were recruited. Peripheral venous blood samples were drawn from the infarct patients 1, 7, 14, and 21 days after onset. Monocytes were isolated from peripheral blood at those times. HGF concentrations in serum and in a culture medium of monocytes after incubation for 24 hours (monocyte HGF levels) were measured by enzyme linked immunosorbent assay. Serum HGF and monocyte HGF values within seven days after onset of myocardial infarction were significantly higher than those of control subjects and decreased by day 14. There were significant positive correlations between serum HGF and monocyte HGF levels on day 7; between maximum plasma creatine phosphokinase levels and serum HGF levels on day 1; between maximum plasma C reactive protein and serum HGF levels; and between maximum C reactive protein and monocyte HGF levels. Monocyte HGF levels were raised in the patients with progression of ventricular enlargement in the course of acute myocardial infarction. Early serum HGF concentrations reflect the extent of myocardial damage in acute myocardial infarction patients. Inflammation after acute myocardial infarction is supposed to be involved in enhanced HGF production. Monocytes may play an important role in ventricular remodelling after acute myocardial infarction by releasing the cardiovascular protective mitogen HGF.