Background: This study aimed to examine out-of-pocket expenditure for cancer treatments of hospitalized patients and to analyze changing patterns over time. Materials and Methods: This study examined data of all cancer patients receiving inpatient care from two tertiary hospitals from January 2003 to December 2010. Medical expenditures per admission were calculated and classified into those covered and uncovered by the Korean National Health Insurance (NHI) and co-payment. Results: The medical expenditure per admission increased slowly from 3,455 thousand Korean won (KRW) to 4,068 thousand KRW. While expenditures covered by the NHI have increased annually, co-payments have generally decreased. The out-of-pocket expenditure ratio, which means the proportion of uncovered expenditure and co-payment among total medical expenditure dropped sharply from 2005 to 2007 and was maintained at a similar level after 2007. Medical expenditures, NHI coverage, and the out-of-pocket expenditure ratio differed across cancer types. Conclusions: It is necessary to continually monitor the expenditure of uncovered services by the NHI, and to provide policies to reduce this economic burden. In addition, an individual approach considering cancer type-specific characteristics and medical utilization should be provided.