Alteplase (recombinant tissue-type plasminogen activator (rt-PA)) was infused within four hours of onset of symptoms in 286 patients with acute myocardial infarction. Delayed coronary angiography was performed 72 hours after admission with coronary angioplasty if indicated. Electrocardiographic monitoring was continuous during the first hour of treatment. The sum of the ST segment elevations (sigma ST) was calculated on electrocardiograms recorded at entry and an hour later. ST elevations resolved rapidly within one hour of treatment in 189 patients and persisted in 97 patients. Rapid resolution of ST elevation correlated with angiographic coronary patency as determined by coronary angiography 72 hours after admission. The patients with rapid resolution of sigma ST had significantly smaller infarcts and a better clinical outcome than the patients with persistent ST elevation. sigma ST values at entry and one hour after treatment had no additional independent predictive value. Rapid resolution of ST elevations in patients undergoing thrombolysis with alteplase was associated with a significantly smaller release of creatine kinase, better preservation of left ventricular function, lower morbidity, and less short and long term mortality. Rapid resolution of sigma ST elevation is an efficient indicator of clinical outcome in groups of patients with acute myocardial infarction undergoing thrombolysis with alteplase.