OBJECTIVES: A major component of the economic burden associated with the treatment of arthritis relates to surgical joint replacement of the hips or knees. The objective of this study is to compare the burden of hip/knee replacements performed in the U.S. in the year 2000 and 2004. METHODS: The Healthcare Cost and Utilization Project –National Inpatient Survey” (HCUP-NIS) was analyzed to estimate the changes in the burden of hip and knee replacements in the U.S. The HCUP itself is a compilation of billing data which can yield estimates of national hospital charges as well as the total number of hip and knee replacement procedures. In 2004, the HCUP-NIS database contained nearly 8 million records from about 1,000 hospitals. The ICD-9-CM procedure codes used in identifying patients are 81.51 (total hip replacement) and 81.54 (Total/partial knee replacement). RESULTS: In the year 2004, approximately 225,900 total hip replacement and 431,458 total knee replacement procedures were performed in the U.S. This is a 37% increase in hip replacement procedures and a 53% increase in total knee replacements compared to the year 2000 (164,458 total hip replacement and 281,534 total knee replacement). In terms of economic burden, the national bill of hospital charges increased by 111% (from $10.8 billion in year 2000 to $22.9 billion in year 2004). CONCLUSION: The data shows a steep increase in the burden of joint replacements. Not considering pre and post-operation care, the estimated hospital charges for joint replacements alone were nearly 22.9 billion dollars in the United States during the year 2004. This data, however, does not provide reasons of steep increase in the burden of joint replacements during this four year period. Further study is needed to address these issues.