To investigate the clinical significance of plasma brain natriuretic peptide (BNP) measurement in patients with acute myocardial infarction (MI), circulating levels of BNP, atrial natriuretic peptide, creatine kinase (CK), and hemodynamic parameters were serially determined in 24 patients with a first episode of acute MI. Plasma BNP (mean ± SEM) gradually increased and peaked 21 h after the onset (from 13.7 ± 2.2 to 23.0 ± 3.3 fmol/ml; p < 0.001). A significant correlation was found between the increase in plasma BNP level and both the peak CK level (r = 0.83; p < 0.05) and the MI size (r = 0.74; p < 0.05). The increase in plasma BNP in the acute phase was found to be a significant predictor of left ventricular (LV) function evaluated in the convalescent phase (LV ejection fraction, r = -0.63; p < 0.05, LV end-diastolic pressure, r = 0.56; p < 0.05). In conclusion, in patients with acute MI, increases in plasma BNP concentration during the early phase reflect MI size, and thereby may predict later LV function.