The aim of the study was to measure plasma neuropeptide Y (NPY) in patients with acute myocardial infarction (AMI). Eighteen patients (13 females and 5 males; range 55 to 75 years) admitted both early (n 12; mean preadmission time 3 +/- 2 hours) and late (n 6; mean preadmission time 25 +/- 16 hours) to Coronary Care Unit were studied. Plasma NPY values significantly above normal range (15-25 pg/ml) and early admitted patients (26 +/- 10 pg/ml) have been found in late admitted patients (72 +/- 28 pg/ml; p < 0.05) both on the admission and during the stay in the Coronary Care Unit (96 hours). In early admitted patients no significant correlation was found between both AMI localization and the thrombolytic treatment. A significant increase in plasma NPY levels has been observed in all patients with complicated myocardial infarction (both in subclinic and clinic heart failure or extension of infarct size). According with the literature data our results have shown a significant correlation between increase of plasma NPY levels and failure of myocardial pump. Moreover, the plasma NPY increase which has been observed in preclinic heart failure when plasma catecholamine levels is usually not significantly different from normal values, may be a useful and reliable prognostic marker.