Abstract Thrombin- and plasmin-related indices were measured in peripheral blood of 50 patients at the onset of acute myocardial infarction (AMI) and 35 chronic stable angina (CSA) patients to establish which are most indicative of acute occlusive coronary thrombosis. AMI compared with CSA had significantly higher plasma prothrombin fragment F1.2, thrombin-antithrombin III (TAT) and plasmin-antiplasmin (PAP) complexes (direct indices), but similar soluble fibrin and D-dimer levels (indirect indices). F1.2, TAT and PAP were > the 90th percentile of CSA levels in 15, 10 and 29 AMI patients, respectively. Thus, direct indices of thrombin and plasmin formation appear more indicative of acute coronary thrombosis than indirect ones. In AMI, raised F1.2, TAT and PAP in only some patients is consistent with wide variability in the activity of the underlying thrombotic process.