Interleukin-1β (IL-1β ) is known to have a number of effects on the different cell types present within coronary arteries. In this study we identified the location and phenotype of cells containing IL-1β in human coronary artery specimens from patients suffering from either coronary atherosclerosis or cardiomyopathy and correlated the presence of IL-1β with disease severity. Luminal endothelial cells, adventitial vessel wall cells, and macrophages were double labeled immunohistochemically for IL-1β protein and a cell type–specific monoclonal antibody for either endothelial cells or macrophages. In situ hybridization was performed to locate the presence of IL-1β mRNA within the coronary artery wall. In this study IL-1β protein was found to be increased in the adventitial vessel walls of atherosclerotic coronary arteries compared with coronary arteries from nonischemic cardiomyopathic hearts. This increase was directly proportional to the severity of coronary atherosclerosis. IL-1β protein was also detected in luminal endothelium and macrophages of atherosclerotic coronary arteries and coronary arteries from nonischemic cardiomyopathic hearts. IL-1β mRNA was found in luminal endothelial cells, adventitial vessel endothelial cells, and macrophages. We conclude that IL-1β is produced by endothelial cells and macrophages in coronary arteries from ischemic hearts and to a lesser extent from nonischemic cardiomyopathic hearts.