This study examines the effect of coronary angioplasty on the progression and appearance of new disease in sites of the coronary tree that were not dilated by the balloon. We examined 355 pairs of coronary angiograms from 252 patients. The study consisted of consecutive patients who were referred for catheterization >1 month after successful angioplasty. Progression/ regression and the appearance or new narrowings at sites not dilated by angioplasty were determined. The life-table method was used to determine outcome, and any event (progression, regression, and new narrowing) was analyzed according to the time of occurrence. The angioplasty artery was compared with the non-angioplasty artery and the effect of restenosis was determined by comparing arteries with and without restenosis. Progression/regression rates were not significantly different in angioplasty and non-angioplasty arteries. More new narrowings were identified in the angioplasty artery (p < 0.01). With regard to narrowings located in the angioplasty artery, progression was more common, regression less common, and the appearance of new narrowings more common in arteries with restenosis than in non-angioplasty arteries or arteries without restenosis. We believe that mechanical trauma to the artery during angioplasty could accelerate disease progression and the appearance of new narrowings in angioplasty arteries, whereas normalization of flow rate and pattern, especially in arteries without restenosis, attenuates the rate of progression and the appearance of new narrowings in these arteries. The final outcome depends on the balance between these factors.