Objectives We evaluated the association between guiding catheter size and complications of percutaneous coronary intervention (PCI). Background The association between guiding catheter size and complications of PCI in contemporary practice remains controversial. Methods Procedure and outcome variables from 103,070 consecutive patients that underwent PCI with 6-F (n = 64,335), 7-F (n = 32,676), and 8-F (n = 6,059) guide catheters were compared. Results Compared with 6-F guides, PCIs performed with 7- and 8-F guides were associated with incrementally more contrast agent use, and more post-PCI complications including contrast-induced nephropathy, vascular access site complications, bleeding, transfusion, major adverse cardiac event, and death. After multivariate analysis, the use of larger guides were associated with a higher risk of contrast-induced nephropathy (7-F odds ratio [OR]: 1.18, p = 0.0004; 8-F OR: 1.44, p < 0.0001), vascular complications (7-F OR: 1.19, p = 0.0002, 8-F OR: 1.68, p < 0.0001), decline in hemoglobin >3 g/dI (7-F OR: 1.12, p < 0.0001, 8-F OR: 1.72, p < 0.0001), and post-procedure blood transfusion (7-F OR: 1.08, p = 0.03; 8-F OR: 1.80, p < 0.0001), whereas major adverse cardiac events (7-F OR: 1.06, p = 0.13; 8-F OR: 1.37, p < 0.0001) and in-hospital mortality (7-F OR: 1.11, p = 0.13; 8-F OR: 1.34, p = 0.03) were increased with 8-F but not 7-F guides. Conclusions Compared with 6-F guides, PCIs performed with 7- and 8-F guides were associated with more contrast medium use, renal complications, bleeding, vascular access site complications, greater need for post-procedure transfusion, and 8-F guides with increased nephropathy requiring dialysis, in-hospital major adverse cardiac events, and mortality. These data suggest that selection of smaller guide catheters may result in improved clinical outcome in patients undergoing contemporary PCI. (J Am Coll Cardiol Intv 2009;2:636-44) (C) 2009 by the American College of Cardiology Foundation