Recent research suggests that inflammatory indexes are increased in unstable angina; however it is not known if they reflect inflammatory activity in the coronary arteries or the effects of recurrent ischemia On the myocardium. To address this issue, in 20 pts with unstable angina (defined as recurrent chest pain at rest associated with transient ST-T changes) blood samples were simultaneously taken from the coronary sinus (CS), from aorta (Ao) and from the coronary artery (CA) just distally to the culprit lesion. Surface expression of neutrophils (PMN) and monocytes (MONO) CD11B/CD18 adhesion receptors was detected by direct immunofluorescence evaluated by flow cytofluorimetry using monoclonal antibodies tagged with fluorescent markers. Results are given as molecular equivalents of soluble fluorescein. Ao CA CS PMN 28.5 ± 4.1 23.1 ± 32 37.3 ± 5.1 * † MONO 23.7 ± 3.9 27.6 ± 4.6 33.1 ± 4.8 § * p < 0.02 CS vs Ao † p < 0.0.2 CS vs CA § p < 0.02 CS vs AO These data show that in patients with unstable angina a significant upregulation of CD11B/CD18 adhesion receptors of neutrophils and monocytes occurs in the coronary sinus as compared to the aorta and the culprit lesion site suggesting that leukocyte activation takes place at the microcirculatory level and not within the epicardial coronary arteries.