1. Adrenomedullin, a newly identified vasorelaxant peptide, participates in the regulation of the cardiovascular system. To investigate the pathophysiological significance of adrenomedullin in patients with acute myocardial infarction, we measured plasma levels of adrenomedullin. 2. Cardiac catheterization was performed on admission, after 1 day, and after 4 weeks in 36 patients with acute myocardial infarction. We measured plasma levels of adrenomedullin, atrial natriuretic peptide and brain natriuretic peptide in the right atrium, pulmonary artery and aorta. 3. Plasma levels of adrenomedullin in the right atrium (mean +/- SEM) were significantly increased on admission (4.2 +/- 2.6 h) in patients with acute myocardial infarction (10.6 +/- 1.0 pmol/l) compared with controls (5.2 +/- 0.3 pmol/l, P < 0.01). In addition, plasma levels of adrenomedullin were further elevated in patients with congestive heart failure (12.3 +/- 1.4 pmol/l) compared with patients without congestive heart failure (7.8 +/- 0.6 pmol/l, P < 0.01). In patients with congestive heart failure, plasma adrenomedullin on admission significantly correlated with atrial natriuretic peptide and brain natriuretic peptide. 4. These results suggest that plasma adrenomedullin increases in the early phase of acute myocardial infarction and that volume expansion may be one of the additional stimuli for the release of adrenomedullin in patients with acute myocardial infarction complicated by congestive heart failure.