BACKGROUND : Although the manufacturer recommends that excessive speed reduction (>5000 rpm) be avoided during rotational atherectomy (RA) for safety, excessive speed reduction is sometimes observed in clinical practice. The purpose of the present study was to examine the factors associated with excessive speed reduction during RA. METHODS : Lesions (n = 300) treated by RA were divided into 3 groups: a mild speed reduction group (≤5000 rpm) (n = 182), a moderate speed reduction group (>5000-≤10,000 rpm) (n = 97), and a severe speed reduction group (>10,000 rpm) (n = 21). Two multivariate logistic regression analyses was performed to investigate the factors associated with >5000 rpm speed reduction, and factors associated with >10,000 rpm speed reduction. RESULTS : Multivariate logistic regression analysis revealed that an ostial right coronary artery (RCA) lesion (OR 6.13, 95% CI 1.82-20.70, P = 0.004) and total ablation time (every 10 s increase: OR 1.09, 95% CI 1.05-1.14, P < 0.001) were significantly associated with >5000 rpm speed reduction. An ostial RCA lesion (OR 14.17, 95% CI 3.85-52.23, P < 0.001), use of intra-aortic balloon pump support (OR 4.19, 95% CI 1.18-14.87, P = 0.03), and systolic blood pressure just before RA (every 10 mmHg increase: OR 1.25, 95% CI 1.02-1.52, P = 0.03) were significantly associated with >10,000 rpm speed reduction. CONCLUSIONS : RA of ostial RCA lesions was significantly associated with excessive speed reduction during RA, which implies that RA of ostial RCA lesions is technically more difficult than RA of non-ostial RCA lesions.