The usefulness of QRST time-integral (IQRST) values of 12-lead electrocardiograms for diagnosing a prior myocardial infarction complicated by left bundle branch block (LBBB) was determined. The study consisted of 25 patients with LBBB (11 with and 14 without myocardial infarction). The IQRST values in each lead point of 12-lead electrocardiograms were calculated. Data from 607 normal subjects were used as controls and mean +/- 2 standard deviations was regarded as the normal range. The following parameters were derived: number of leads less than the normal range of IQRST values (nQRST) and sum of the differences between the normal mean IQRST value and IQRST value of a given patient in leads where this value was less than the normal range (sigma QRST). The criteria of nQRST (12-lead) greater than or equal to 5 and sigma QRST (12-lead) greater than or equal to 500 microV in 12-lead electrocardiograms were selected on a relative cumulative frequency distribution and demonstrated the presence of a myocardial infarction in LBBB with a sensitivity of 82% and a specificity of 100% for each. With regard to the localization of the myocardial infarction, the criterion of sigma QRST (V1-6) greater than or equal to 300 microV in leads V1-6 of 12-lead electrocardiograms demonstrated the presence of an anterior myocardial infarction in the LBBB with a sensitivity of 88% and a specificity of 77%. It was difficult to localize an inferior myocardial infarction in patients with LBBB by using IQRST values of inferior leads.(ABSTRACT TRUNCATED AT 250 WORDS)