Interleukin-6 (IL-6) plays a key role in the synthesis of human acute-phase protein and several acute-phase responses occur in patients with acute myocardial infarction (AMI). We examined the plasma levels of IL-6 in 23 consecutive patients with AMI over the course of 4 weeks and in 30 control subjects. In patients with AMI, the plasma IL-6 levels (in picograms per milliliter) were increased at all sampling points from admission to discharge (ranging from 28.5 +/- 6.6 to 46.5 +/- 7.8) compared with levels in control subjects (11.4 +/- 2.9; p < 0.01). Cardiac catheterization did not influence plasma IL-6 levels. The plasma IL-6 level reached its peak approximately 3 days (46.5 +/- 7.8) and approximately 1 week after admission in patients with AMI. There was a significant positive linear correlation between the peak level of plasma IL-6 minus the level on admission and the peak level of plasma C-reactive protein in patients with AMI. The peak IL-6 level did not correlate with the peak levels of creatine kinase, pulmonary capillary wedge pressure, or left ventricular ejection fraction at 4 weeks. We conclude that the plasma IL-6 level is increased over a time course of 4 weeks in patients with AMI.