We evaluated a new sensitive creatine kinase (CK) isoenzyme MB immunoassay with regard to clinical applicability and clinical outcome in 156 patients admitted consecutively to the Coronary Care Unit and suspected of having acute myocardial infarction (AMI). Sixty-five patients (42%) had AMI based on WHO criteria; 65 (42%) had ischemic heart disease (IHD) without AMI, and 26 (16%) had non-IHD. The 65 IHD-patients without AMI could be subdivided into a group of 24 patients with significant changes in serum CK MB levels and 41 patients with stable serum CK MB levels as compared to the non-IHD group. The cumulative cardiac survival after 2 1/2 year (i.e. not suffering cardiac death) was 95 +/- 3% for patients with stable CK MB levels; for those with changes in CK MB levels it was 66 +/- 10% (p < 0.003). It was 52 +/- 6% for the patients with AMI, similar to patients with changes in CK MB levels (p = 0.15). We conclude, that this new CK MB assay can, in patients with IHD without AMI, detect a sub-group, representing one-third of the patients in this group, which is not diagnosed using routine diagnostic procedures. These patients are characterized by a poor clinical outcome, similar to patients with definite AMI.