The Benchmark intra-aortic balloon counterpulsation (IABC) registry maintains prospectively gathered clinical information on a large cohort of IABC patients. The purpose of the present report is to compare in-hospital outcomes and complications in patients treated with the newer 8 vs. 9.5 Fr size catheters. Between January 1997 and August 2000, data on 7,078 9.5 Fr and 2,254 8 Fr IABC insertions were submitted to Benchmark. This was not a randomized comparison but rather a posthoc analysis of prospectively gathered data. There was less limb ischemia with the 8 Fr IABC size catheter. There were no significant differences in bleeding or mortality between the two groups. Smaller IABC catheter size is associated with significantly less limb ischemia, especially in higher-risk patients. The large, population-based, ongoing Benchmark registry provides a useful vehicle for outcomes research concerning the evolving practice of IABC.