In this study, which includes 101 patients with acute ST segment-elevated myocardial infarction, we investigated the influence on the increased coagulation activity after streptokinase treatment by adding low-molecular-weight (LMW) heparin or placebo and the relation between the coagulation activity and ischemic episodes, coronary patency, and mortality. The expected increase of prothrombin fragment 1+2 (F1+2), thrombin-antithrombin (TAT), and D-dimer were significantly attenuated at 2, 6, and 18 h (D-dimer only at 18 h) in the dalteparin group compared to placebo. Ischemic episodes during the first 24 h appeared significantly more often in patients with F1+2 levels above the median at 18 h. There was a tendency to a lower frequency of Thrombolysis In Myocardial Infarction Trial (TIMI) grade 3 flow in the infarct-related artery in patients with TAT and D-dimer levels above the median at 18 h. F1+2, TAT, and D-dimer were significantly higher after 18, 6, and 18 h, respectively, in the deceased compared to surviving patients. Also, the lack of reduction of the levels of F1+2 between 6 and 18 h was related to a raised mortality. In conclusion, adjuvant treatment with LMW heparin to streptokinase attenuates increased coagulation activity. This might be of importance as remaining high coagulation activity is associated with signs of early reocclusion and raised mortality.