The initial electrocardiogram is crucial in accurately selecting patients with chest pain for thrombolytic therapy. An electrocardiogram with a large amount of ST-segment elevation and depression is "visually alarming," and therefore, may influence the efficiency of patient treatment with thrombolytic therapy. It was hypothesized that the amount of ST-segment deviation present on the initial electrocardiogram was an important variable in determining the time to initiation of thrombolysis in the emergency department. The time from arrival at the emergency department to thrombolysis was measured in 93 consecutive patients with suspected acute myocardial infarction (AMI) who were treated with intravenous thrombolytic therapy by emergency department physicians. This was correlated with the sum of ST-segment elevation and depression present on the initial electrocardiogram. AMI was proved in 83 patients (89%). In patients with proved AMI, the average time to thrombolysis was 50.8 +/- 25.6 minutes. Treatment began within the goal of < or = 30 minutes in 18 patients (22%) and was excessively delayed at > or = 60 minutes in 24 (29%). Regression analysis of multiple clinical variables revealed that ST-segment sum was the only variable that significantly influenced the time to thrombolysis (r = -0.42; p < 0.001). For patients treated in < or = 30 minutes, the average ST-segment sum was 21.1 +/- 13.5 vs 11.5 +/- 11.4 mm for those treated in > or = 60 minutes (p = 0.01). In 10 patients mistakenly treated with thrombolytic therapy, the electrocardiographic processes responsible for ST-segment elevation included the early repolarization variant, left ventricular hypertrophy, old anterior AMI with persistent ST-segment elevation, and conduction delay.(ABSTRACT TRUNCATED AT 250 WORDS)