['In order to detect statistically relevant differences in mortality rates, it is essential to have adequate sample sizes and event rates. Our hypothesis is that case volumes and mortality rates present in pediatric cardiac surgery are too low to allow the use of mortality to differentiate between hospitals.', 'Pediatric cardiac surgical operations performed at U.S. hospitals were identified in the Nationwide Inpatient Sample (NIS) Database 2000 to 2005 (21,709 operations from 161 hospitals). Hospital annual surgical volumes and in-hospital mortality rates for Risk Adjustment for Congenital Heart Surgery, version 1 (RACHS-1) categories and select individual operations were calculated. The actual case volumes were compared with thresholds necessary to detect a doubling and a 5 percentage point increase in the mortality rate.', 'No hospital had a sufficient annual case volume to determine a doubling of or 5 percentage point increase in mortality for any individual operation and a minority (0% to 5.6%) had sufficient volume to detect these differences for specific RACHS-1 categories. Minimum hospital case volumes needed to detect a doubling of mortality from a benchmark ranged from 11 for RACHS-1 category 5 to 2,935 for RACHS-1 category 1. Minimum case volumes necessary to detect a 5 percentage point difference in mortality between two hospitals ranged from 173 for RACHS-1 category 1 to 1,483 for RACHS-1 category 5. Five hundred twenty-five patients were needed to detect a doubling of overall hospital mortality rate compared with another hospital. Only 1.6% (n = 4) of hospitals met this minimum caseload.'] Is mortality rate a valid indicator of quality differences between pediatric cardiac surgical programs?