Background Early meta-analyses suggested that prophylactic lidocaine use reduces ventricular fibrillation but increases mortality rates after acute myocardial infarction. We determined the frequency and effect on clinical outcomes with its use in the thrombolytic era. Methods and Results We studied 43,704 patients enrolled in GUSTO-I or GUSTO-IIb who had ST-segment elevation, underwent thrombolysis, and survived at least 1 hour after enrollment. Odds ratios (OR) and confidence intervals (CI) were calculated for the risk of asystole, atrioventricular block, ventricular fibrillation, and ventricular tachycardia during hospitalization; for 24-hour, in-hospital, and 30-day mortality rates; and for 24-hour and 30-day mortality rates after adjustment for baseline predictors of death. In GUSTO-I and GUSTO-IIb, 16% and 3.5% of patients, respectively, received prophylactic lidocaine. They had a lower risk of death at 24 hours (OR 0.81, 95% CI 0.67 to 0.97) and trends toward lower odds of in-hospital death (OR 0.90, 95% CI 0.81 to 1.01) and death at 30 days (OR 0.92, 95% CI 0.82 to 1.02). After adjustment for baseline characteristics, however, the odds of death were similar with or without lidocaine (OR 0.90 and 0.97, respectively). Outside the United States, lidocaine was associated with higher incidences of all serious arrhythmias, but in US patients it conferred a lower likelihood of ventricular fibrillation and no increase in asystole, atrioventricular block, or mortality rates. Conclusions Prophylactic lidocaine use has decreased with the advent of thrombolysis, although its use may not be associated with increased mortality rates. (Am Heart J 1999;137:799-805.)