OBJECTIVE: Based on the empirical data of Taiwan's National Health Insurance (NHI) program, this study intends to analyze health care utilization at the individual level among five quintiles and to test the following hypotheses: a) Per capita medical care expenditure (MCE) increases with income, and b) Under-utilization among dependent people (under age 14 and over 65) occurs primarily in the lower income quintile. METHODS:Since 1995, a compulsory social health insurance scheme is implemented and has covered about 97% of the total Taiwan population. This study linked the survey of family income and expenditure data (year 2004) from the Directorate-General of Budget Accounting and the insurance claim data (2004) from the Bureau of NHI by identification number to analyze the relationship between MCE and income.RESULTS:There are 91.8% beneficiaries who used at least once outpatient service which accounts for 73% of total MCE, and 8.02% beneficiaries who used inpatient service. The per capita poorest spent the largest (US$ 623) MCE for outpatient service, and the per capita richest spent the largest (US$ 2306) MCE for inpatient service. The MCE disparity ratio of inpatient and outpatient service from the poorest to the riches was 2.6, 3.5, 3.8, 5.2 and 5.6, respectively. After age 60, the poorest and richest quintile spent 74.1% and 38.5% of total inpatient MCE among five quintiles. Under age 45, the per capita inpatient MCE increases with income. The gender ratio of inpatient service from the poorest to the richest was 1.21, 1.09, 0.95, 0.96 and 0.99, respectively. CONCLUSIONS:The poorest suffered from Diabetes Mellitus, cardiovascular diseases and liver cirrhosis.