This study compares the complication rates of patients undergoing rotational atherectomy of the left coronary system who had either minimal or significant narrowing of the right coronary artery (RCA). A series of 1,872 patients from a multicenter registry who were treated for left coronary artery disease were divided into <70% diameter stenosis (mild) and > or = 70% stenosis (severe) of the RCA. The patient demographics, lesion characteristics, and frequency of procedural complications for each group were compared. Of the 1,872 patients undergoing rotational atherectomy of the left coronary system, 86.3% (n = 1,616) had mild RCA disease and 13.7% (n = 256) had severe RCA disease. Comparing the mild and severe groups, death (0.8% vs 3.1%, p <0.005), non-Q-wave myocardial infarction (5.1% vs 8.6%, p <0.04), and bypass surgery (2.7% vs 5.8%, p <0.02) were increased in the severe group. Within the severe group, 7 of 8 deaths were in the 128 patients with total occlusion of the RCA. Multivariate analysis demonstrated that RCA stenosis increases the risk of death by 4.9, bypass surgery by 2.6, and non-Q-wave myocardial infarction by 1.8. Patients treated for left coronary disease who have > or = 70% stenosis of the RCA have increased complications during rotational atherectomy.