Serum cardiac troponin I-values were compared to conventionally obtained diagnosis in 319 consecutive patients suspected of having myocardial infarction, of which 46 patients were given this diagnosis. All patients with troponin I > 20 micrograms/l (n = 40) also had abnormal creatine kinase and abnormal creatine kinase isoenzyme MB activity. All patients with troponin I values in the range 1.0-19.9 micrograms/l (n = 50) had a diagnosis of heart disease (myocardial angina pectoris, myocardial infarction, arrythmia, heart insufficiency). In this patient group, the creatine kinase measurements showed pathological values in only 12 cases. Troponin I seems to be a sensitive indicator of cardiac cell injury, and measurements of troponin I seems to be useful in ruling out cardiac injury.