Objective To study the relationship of the plasma concentration of brain natriuretic peptide (BNP) with severity, left ventricular function, prognosis in the patients with acute myocardial infarction(AMI).Methods 63 patients with AMI were enrolled in this clinical investigation. The plasma concentrations of BNP were measured at 24 hours after the infarction. According to the electrocardiogram, the infarction areas were measured. Left ventricular function was evaluated by echocardiography with the parameters of left ventricular ejection fraction (LVEF) after one week. Patients were followed up for three months, relationship of plasma concentration of BNP with infarction areas, left ventricular function and prognosis in the patients of AMI were observed. Results The concentration of BNP in the patients with AMI was related to severity of the disease. With enlargement of infarction areas, the BNP increased (P0.01) .With the decrease of LVEF, the BNP increased (P0.05) .The clinical cardiac events occurred more frequently in the patients with a higher BNP(P0.05). Conclusion Plasma concentration of BNP in patient with AMI might be a reliable biochemical marker which can objectively evaluate the degree and prognosis of this disease.