We report on the first detailed comparison of evaluation results regarding the correct billing in the G-DRG (German diagnosis-related group) system. For two Medical Review Boards of the Statutory Health Insurance Funds of comparable size (MDK Baden-Württemberg and MDK Westfalen-Lippe), we analysed consecutive expertises regarding correct billing according to section sign 275 SGB V, and the results were compared in terms of the frequency of DRG-relevant error codes, their relevance to revenue, and the question of error clustering (specific DRGs, primary diagnoses, etc.). The analysis comprised 51,010 individual expertises pertaining to billings of the year 2005 (admittance to hospital from January 1 to December 31, 2005). The proportion of disapproved cases was 38.5% in Baden-Württemberg and 44.6% in Westfalen-Lippe. Among these, errors to the disadvantage of the Health Insurance (incorrectly high) were 33.9% and 39.3%, respectively, and errors to the disadvantage of the hospitals (incorrectly low) were 4.6% and 5.3%, respectively. The resulting ratio (incorrectly high vs. low) was an identical 7.4 in both cases. Not only the most commonly rejected DRGs but also the primary and secondary diagnoses were similar in both cases, while the disapproved procedure codes showed a significant variability (analysis based on the respective 10 most common objections). We discuss the similarities and differences in these results and their possible causes, and demonstrate the cost relevance of this audit segment. Result comparisons of this type can yield insights into streamlining of the review practice of Medical Review Boards, as well as increase the efficiency and effectiveness of the selection of cases.