Purpose: This study examined potential determinants of gender differences in utilization of health care services among Korean adults. Methods: The study population was 21,647 adults ${geq}$25-years-of-age who had responded to a health interview survey conducted as part of the 2005 National Health and Nutrition Surveys. Relative gender differences in the use of each health service were assessed using chi-square test and sex ratios. The contribution of potential factors of sex differences in the use of health services was evaluated by comparing the odds ratio and sex ratio before and after adjustment for such variables. Results & Conclusions: More females had visited a physician and been admitted to hospital, but hospitalization time was longer for males. Adjustment for poor self-rated health, number of chronic disease and limit of full term for ADL led to a reduction in the odds ratio of females compared to males for health service utilization. However, adjustment for socioeconomic factors (household income, education, occupation, and health insurance) magnified the gender difference concerning length of hospitalization. Factors that explain gender-related differences in utilization of health care services are concluded to be different health needs and socioeconomic status.