Background Biochemical markers have been suggested for the noninvasive diagnosis of reperfusion early after thrombolysis. Their ability to discriminate between Thrombolysis in Myocardial Infarction (TIMI) 2 and 3 flow grades remains unknown. Methods In 97 patients with myocardial infarction ≤6 hours, myoglobin, troponin T, MBCK, and MMCK isoforms were measured before thrombolysis and after 90 minutes. A 90-minute coronary angiography ascertained the patency status of the infarct-related artery in all patients according to the TIMI grade flow (group A: TIMI 0-1 (n = 35), group B: TIMI 2 (n = 17), and group C: TIMI 3 (n = 45). For each marker the absolute rate of increase and the relative increase 90 minutes after thrombolysis were studied. Results Both absolute values and absolute rates of increase at 90 minutes of myoglobin were higher in group B than in groups A and C (p < 0.05, not significant for other markers). Relative increases were consistently higher in group C than in other groups with statistical significance for myoglobin in the subset of patients treated for >3 hours. Conclusion Diagnostic indexes based on relative increases tend to discriminate between patients with TIMI 2 and 3 flow, and the best performance is obtained when the relative increase of myoglobin at 90 minutes is used in patients treated later than 3 hours after onset of symptoms. (Am Heart J 1997;134:1044-51.)