Results:Although the transition from the ICD-9-CM to the ICD-10 version of MS-DRGs resulted in 1.68 percent of the patients being assigned to a different MS-DRG, payment increases and decreases due to the changes in MS-DRG assignment essentially netted out, resulting in a minimal impact on aggregate payments to hospitals (+0.05 percent) and on the distribution of payments across hospital types (-0.01 to +0.18 percent). Mapping ICD-10 data back to ICD-9-CM, and using the ICD-9-CM MS-DRGs, resulted in 3.66 percent of patients being assigned to a different MS-DRG, a modest decrease in aggregate payments to hospitals (-0.34 percent), and modest changes in the distribution of payments across hospital types (-0.14 to -0.46 percent). Discussion:As demonstrated by MS-DRGs, a direct conversion of an application to ICD-10 can produce consistent results with the ICD-9-CM version of the application. However, the use of mappings between ICD-10 and ICD-9-CM will produce less consistent results, especially if the mapping is not tailored to the specific application.