Plaque disruption with thrombus (T) is the major pathogenetic mechanism for the acute coronary syndromes of myocardial infarction and unstable angina (UA). Recently, clinical studies have suggested a role for inflammation in UA. The relationship between inflammation and thrombus in UA has not been well studied. Analysis of tissue from directional atherectomy specimens can be used to correlate inflammation and thrombus. Thus, we analyzed the results of immunohistochemical staining for macrophages (KP-1) in 54 culprit lesions (19 with rest angina, 15 with new onset/crescendo, 10 post-infarction and 10 with stable angina. Thrombus was detected by H and E staining. KP-1 staining was graded as 0 to 2+ and 2+ was considered positive(+). Pathologic specimens were analyzed blindly.Thrombus was found predominantly in rest angina and post-Infarction. KP-1 staining was more common in all syndromes other than stable. A positive KP-1 was significantly associated with thrombus particularly in rest angina and post-infarction where 14 of 17 KP-1 positive lesions were positive for thrombus. Thus, these data suggest a strong association between inflammation and thrombus in unstable rest angina and post-infarction.