Objectives: The purpose of this study was to analyze the effect of outpatient cost-sharing on health care utilization by the elderly. Methods: The data in this analysis was the health insurance claims data between July 1999 and December 2008 (114 months). The study group was divided into two age groups, namely 60-64 years old and 65-69 years old. This study evaluated the impact of policy change on office visits, the office visits per person, and the percentage of the copayment-paid visits in total visits. Interrupted time series and segmented regression model were used for statistical analysis. Results: The results showed that outpatient cost-sharing decreased office visits, but it also decreased the percentage of copayment-paid visits, implying that the intensity of care increased. There was little difference in the results between the two age groups. But after the introduction of the coinsurance system for those patients under age 65, office visits and the percentage of copayment-paid visits decreased, and the 60-64 years old group had a larger decrease than the 65-69 years old group. Conclusions: This study evaluated the effects of outpatient cost-sharing on health care utilization by the aged. Cost sharing of the elderly had little effect on controlling health care utilization