OBJECTIVE To evaluate the clinical usefulness of leukocyte elastase concentration for diagnosis of coronary artery disease. BACKGROUND Recent research has shown the important role elastase, a proteolytic enzyme released by neutrophils, in the pathogenesis of coronary atherosclerotic disease. METHODS 95 patients underwent coronary angiography during investigation of chest pain and/or heart valve disease; 38 had normal coronary arteriograms (Group I) and 57 had coronary lesions (Group II). The patients were characterized as regards presence or absence of stable or unstable angina, family history of coronary artery disease, smoking, diabetes mellitus, hypertension, leukocyte counts, plasma lipid and elastase concentrations. Among Group II patients, those with simple atheromatous plaques were distinguished from those with complex plaques. RESULTS Elastase concentrations were greater in the Group II than in the Group I (41 +/- 21 vs 27 +/- 14 micrograms/L) (p < 0.001), and greater among complex plaque patients than among simple plaque patients (53 +/- 27 vs 33 +/- 12 micrograms/L) (p < 0.001). Logistic regression analysis showed than elastase concentration, angina, age and sex had independent value for prediction of coronary artery disease and that the risk increased by 7% for every 1 microgram/L increase in elastase concentration. Among Group II patients, the risk of complex plaque was greatest for those with unstable angina and high elastase concentration, increasing by 6% for every 1 microgram/L increase in elastase concentration. CONCLUSIONS Peripheral blood leukocyte elastase concentration is a sensitive diagnostic marker of coronary artery disease. High values suggest the presence of complex atheromatous plaques.