Objectives: Following the implementation of National Health Insurance, high-tech medical devices have become one of the main factors causing an increase in health care expenditure. Among the expensive medical devices, CT and MRI have the highest expenditure. This study investigates CT and MRI utilization and the factors that affect CT and MRI expenditure in Taiwan. Methods: This study used nationwide CT and MRI claim data for the years 1998 to 2001 from BNHI in Taiwan. We analyzed the trend of CT and MRI utilization at outpatient and inpatient services. Multiple regression analysis was the statistical method to determine the factors that influence the expenditure on CT and MRI. Results: In 2001 the rate of CT and MRI utilization for outpatients was 1.38 and 0.45 per 1000 patients, and the rate of inpatient use was 87.28 and 15.96 per 1000 patients. From 1998 to 2001, the annual growth rates in CT and MRI utilization for outpatients were 4.47% and 18.56%, whereas the annual growth rates for inpatients were -2.52% and 4.90% respectively. The rate of CT and MRI utilization for outpatients is still growing at an increasing rate while it become stable for inpatients. The multiple use of CT or MRI procedures in one week has significantly decreased. The CT or MRI-population ratio, hospital-based physician-population ratio, and female ratio have a significantly positive relationship with CT or MRI expenditure after controlling for other factors. Average family income significantly has a negative relationship with CT or MRI expenditure. However, the proportion of pediatric (≤14 years) population and aged (≥65 years) population didn't significantly influence CT or MRI expenditure after controlling for other factors. MRI and its utilization increase rapidly in areas with a low MRI-population ratio, whereas increases in CT units and utilizations have distinctly lower rates. Conclusion: Increases in supply-side health care resources have had more impact on the growth in CT and MRI expenditure than that of demand side. However, changes in the population's age structure will not significantly increase CT or MRI expenditures. The utilization review policy was an effective control on the multiple use of CT or MRI.