The purpose of this study was to explore the possibility of extrinsic coronary artery compression following nonpenetrating cardiac trauma. Direct impact on the anterior surface of the heart was produced by a blunt air-pressurized impactor in 34 open-chest anesthetized dogs. Sections from the left anterior descending coronary artery and the left circumflex coronary artery were obtained for microscopic examination from each of the 17 dogs that survived the duration of observation (90 minutes). Gross examination of the external surface of the heart showed contusions over the anterior surface and prominent extravascular hemorrhage that followed the course of the left anterior descending coronary artery, whereas the posterior surface, including the circumflex coronary artery, showed no contusion or extravascular bleeding. In most instances, light microscopy showed narrowing of the lumen of the left anterior descending coronary artery with concave deformation resulting from compression by blood around the vessel. The cross-sections of the circumflex coronary artery, although sometimes oblong, showed no concave deformation. These observations suggest that extrinsic compression of the coronary artery may cause coronary narrowing following nonpenetrating cardiac trauma.