Adventitial fibroblasts have previously been proposed to be a major constituent of the neointima following coronary balloon angioplasty. The current study utilized the bromodeoxyuridine (BrdU) pulse-chase technique to track adventitial fibroblast migration early after balloon injury in swine. BrdU (30mg/kg), a marker of proliferating cells, was given intravenously 1 or 2 days following balloon angioplasty. For each time point, one animal was euthanized 24 hours following injection to identify the location of the proliferating cells, while a second animal was euthanized 25 days following angioplasty to determine if the proliferating cells migrated to form the neointima. Our results demonstrate that BrdU positive cells were located primarily in the adventitia with all three time points 24 hours following balloon angioplasty. Furthermore, when BrdU was injected on day 1 or 2 only 0.65 ± 0.17% and 1.7 ± .64%, respectively, of neointimal cells were BrdU positive on day 25. In conclusion, these results demonstrate a negligible contribution of coronary adventitial fibroblasts to neointima formation following coronary balloon angioplasty, supporting the concept that the neointima is primarily of smooth muscle cell origin.