Leucocyte migration inhibition in patients with ischaemic heart disease was evaluated as an assay for progressive myocardial damage. Abnormal results were observed in 50% of patients with ischaemic cardiac disease. The prevalence of abnormal leucocyte migration inhibition was unrelated to clinical presentation, extent of coronary artery disease, or degree of impairment of left ventricular function. Six of the eight patients with unstable angina pectoris and abnormal leucocyte migration inhibition developed life threatening cardiac complications in the follow-up period compared with five patients with unstable angina and normal tests who developed no complications. A similar association between abnormal leucocyte migration inhibition and complications was not observed in patients with angina pectoris or previous myocardial infarction. Thus, leucocyte migration inhibition may be useful as a prognostic marker in unstable angina and may be an important additional variable to identify a high risk subset.