The effect of metoprolol on corrected QT interval (QTc) was studied retrospectively in 111 survivors of AMI below 70 years of age. Prior to discharge the patients were stratified by age, infarction size and ventricular arrhythmias and randomized. Metoprolol, 100 mg b.i.d., or placebo were given double-blindly to 59 and 52 patients, respectively. QTc intervals were measured four times prior to randomization and three times during the follow-up year. The highest QTc mean was registered on the second day in the CCU. QTc intervals subsequently decreased significantly in both groups between discharge and the three-month control (p < 0.001). Patients on metoprolol had significantly shorter QTc intervals during the follow-up year than those on placebo (0.394 +/- 0.028 vs. 0.406 +/- 0.034 sec, p < 0.001). The QTc-shortening effect of beta-receptor blockade was most marked in patients with prolonged QTc intervals at discharge. Patients who died suddenly had prolonged QTc intervals prior to discharge. In this group the proposed beneficial effect of beta-receptor blockade on QTc interval cannot be evaluated as most of these patients had died before the first control.