Pericarditis may complicate the early phase of myocardial infarction (MI). It occurs when necrosis involves the epicardial surface. To verify if pericarditis may be regarded as a marker of extensive MI, 60 patients with anterior or inferior MI admitted to the Coronary Care Unit within 6 hours from onset of symptoms, were studied by clinical, electrocardiographic and enzymatic parameters. 20 patients developed left ventricular failure (LVF) assessed by clinical, radiologic and hemodynamic indexes (15 mmHg has been considered the upper normal value for mean wedge pulmonary pressure). 9 of the 11 patients with pericarditis (PP) had LVF, versus 11 of the 49 non PP group (P = 0.002). Life threatening arrhythmias (ventricular tachicardia and fibrillation) appeared in 5 of the 11 PP versus 7 of the 49 non PP group (P = 0.04). No significant difference has been found between the two groups concerning the inhospital mortality. In a follow-up of 3 to 18 months, no difference in mortality was observed, while the functional recovery in the PP group was significantly worse (I and II versus III and IV New York Heart Association classes P = 0.003). Higher sigmaST values were found in precordial maps of the PP group, on admission (P = 0.03). After a deep spontaneous fall, sigmaST showed a reelevation which was similar in the two groups. SigmaR showed a greater % decrease however not statistically significant in PP. Creatinekinase enzymatic infarct size was significantly higher in PP group (P = 0.0002). It is concluded that pericarditis is a clinical marker of extensive MI and may be useful in evaluating prognosis and effectiveness of therapeutic interventions in MI.