One measure of the success of thrombolysis is the early patency status of the infarct-related coronary artery. The Thrombolysis in Myocardial Infarction (TIMI) study group designated patency grades 0 (occluded) or 1 (minimal perfusion) as thrombolysis failure and grade 2 (partial perfusion) or 3 (complete perfusion) as success. To evaluate their true functional significance, perfusion grades were compared with enzymatic and electrocardiographic (ECG) indexes of myocardial infarction in 359 patients treated within 4 h with anistreplase (APSAC) or streptokinase. Serum enzymes and ECGs were assessed serially. Patency was determined at 90 to 240 min (median 2.1 h) and graded by an observer who had no knowledge of patient data. Results for the two drug arms were similar and combined. Distribution of patency was grade 0 = 20%, n = 72; grade 1 = 8% n = 27; grade 2 = 16%, n = 58 and grade 3 = 56%, n = 202. Interventions were performed after angiography but within 24 h in 51% (n = 37), 70% (n = 19), 41% (n = 24) and 14% (n = 28) of patients with grades 0, 1, 2 and 3, respectively. Outcomes were compared among the four patency groups by the orthogonal contrast method. Patients with perfusion grade 2 did not differ significantly from those with grade 0 or 1 in enzymatic peaks, time to peak activity and evolution of summed ST segments, Q waves and R waves (contrast 2). Conversely, comparisons of patients with grade 3 perfusion with those with grades 0 to 2 yielded significant differences for enzymatic peaks and time to peak activity for three of the four enzymes (p = 0.02 to 0.0001) and ECG indexes of myocardial infarction (p = 0.02 to 0.0001) (contrast 3). Thus, patients with grade 2 flow have indexes of myocardial infarction similar to those in patients with an occluded artery (grades 0 and 1 flow). Only early grade 3 flow results in a significantly better outcome than that of the other grades. Because early achievement of grade 2 flow does not appear to lead to optimal myocardial salvage, the frequency of achieving grade 3 perfusion alone may best measure the reperfusion success of thrombolytic therapy.