The aim of this study is to evaluate the relationships between obesity and medical care expenditure among Taiwanese adults and to assess the influence of sex, age and socioeconomic status. Our study sample consisted of 12,250 adults aged 18 years or older from the 2001 National Health Interview Survey (NHIS), who had consented to the linking of their survey responses with their NHI claims records. Obesity was defined by Body Mass Index based on the WHO-Asia Pacific categories. Adjusted expenditure for obese class II and class I men were, respectively, 44.6% (95%CI: 27.1%-68.7%) and 39.5% (95%CI: 39.4%-41.2%) greater than normal weight men. For obese class II and class I women, the adjusted expenditure were, respectively, 93.3% (95%CI: 69.9%-114.6%) and 56.1% (95%CI: 50.4%-61.4%) greater than normal weight women. After adjusting for other factors, higher medical care expenditure was associated with a higher BMI for each age group. The relative magnitude of the association became more apparent as age increased. Annual medical care expenditure increased as the BMI increased among women, which was particularly apparent among low socioeconomic status women. On the other hand, the relationship between BMI and medical care expenditure in men varied by household income. In conclusion, there is a strong positive relationship between higher BMI and increased medical care expenditure and this varies according to sex, age and socioeconomic status. Our findings suggest that projections of future health care costs attributable to obesity will need to take into consideration the demographic make-up of the obese population.