The adventitia undergoes remodeling changes after a deep medial coronary injury. Because this process is associated with the formation of adventitial myofibroblasts, which resemble medial smooth muscle (SM) cells, we have examined myofibroblast involvement in the development of neointima. In a porcine model, severe endoluminal coronary injury resulted in fibroblast proliferation and adventitial remodeling. Significant adventitial responses were associated with increased neointimal formation (P < .01). To examine the contribution of adventitial cells to the development of neointima, proliferating cells were labeled with bromodeoxyuridine (BrdU) at 12 and 24 hours after injury, and their subsequent localization was determined by immunohistochemistry (n = 24). At 2 to 3 days after severe injury, the adventitia contained numerous BrdU-labeled cells (37 +/- 4%), whereas the media demonstrated infrequent labeled cells (4 +/- 1%). Adventitial cells lacked alpha-SM actin and desmin, which distinguished them from medial SM cells. At 7 to 8 days, some labeled cells acquired characteristics of myofibroblasts expressing alpha-SM actin. They were found to translocate to the gap between dissected media and contributed to the formation of neointima (76 +/- 19%). At 18 to 35 days, labeled cells were abundant in the neointima (86 +/- 5%). They showed uniform immunostaining for alpha-SM actin but not for desmin, thereby differing from medial SM cells and blood-borne cells. This study demonstrates translocation of adventitial fibroblasts to neointima, their phenotypic modulation to myofibroblasts, and distinct characteristics of myofibroblasts within neointima after severe endoluminal coronary injury. These findings suggest the significance of vascular fibroblasts in the process of arterial repair.