The objective of the study was to monitor collagen metabolism after thrombolytic therapy. Sequential measurements of serum aminoterminal type-III procollagen propeptide (S-PIIINP) and carboxyterminal type-I procollagen propeptide (S-PICP) were made in 62 patients suspected of acute myocardial infarction and receiving thrombolytic therapy. Regardless of whether acute myocardial infarction was confirmed or not, S-PIIINP increased (94-120%) 4 h after streptokinase therapy (p < 0.02), and decreased during the next 20 h with median values at 24 h still above the baseline (p < 0.02). With confirmed acute myocardial infarction, S-PIIINP increased from 24 h towards a plateau reached at day 2-3 (p < 0.01), with values still elevated at 6 months. No similar biphasic pattern was found for S-PICP, but patients with acute myocardial infarction had S-PICP above baseline at 1, 2, and 6 months (p < 0.05). A less pronounced S-PIIINP increase was noted with tissue-plasminogen activator than with streptokinase. Thrombolytic therapy induces collagen breakdown regardless of whether acute myocardial infarction is confirmed or not. With confirmed acute myocardial infarction collagen metabolism is altered for at least 6 months. Furthermore, fibrin-specific and nonspecific thrombolytic agents appear to affect collagen metabolism differently.