In 671 patients with suspected acute myocardial infarction (MI) and no previous MI, precordial mapping with a grid containing 24 chest leads was performed within a few hours (mean = 4.8 hours; range 0-42 hours) after arrival in hospital and four days later. In 76% of the patients the criteria for definite MI, based on serum enzymes and a 12 lead standard electrocardiogram, were fulfilled. Among patients classified as having no definite MI, 2% had abnormal Q-waves on mapping on the fourth day; among patients classified as having a subendocardial MI (no abnormal Q-waves in the 12 standard leads), 7% had abnormal Q-waves on mapping on the fourth day. In patients who subsequently developed anterior MI, changes in the sum of Q-waves and the sum of R-waves were observed for more than 12 hours after onset of pain. ST-elevations defined from a normal population were seen in the initial recording in 60% of patients with anterior MI. Among those in whom the first recording was performed less than or equal to 4 hours after onset of pain, ST elevation was initially seen in 72%. A positive correlation was observed between the initial ST elevation and severity of chest pain, incidence of congestive heart failure and two-year mortality rate. We thus conclude that some further information regarding presence of Q-waves can be obtained in about 5% of patients with suspected acute MI from an increased number of precordial leads.