OBJECTIVES: The objective of this study was to examine the age-specific relationship between body mass index (BMI), prevalence of metabolic disorders, and utilization of outpatient services. METHODS: Data for this study came from the National Health Interview Survey in Taiwan, which was conducted in 2001. With the consent of the respondents, the interview data were linked to their claims in the National Health Insurance database. The self-reported weight and height were used to calculated BMI. Diseases and utilization of outpatient services were identified from the claims data. RESULTS: A linear trend of prevalence was observed with increments of BMI. The same trend was observed for the number of visits to outpatient clinics. The BMI-related medical expenditures did not reach statistical significance. Nevertheless, age was an important factor. After controlling for the number of chronic diseases, the relationship between BMI and utilization of outpatient services disappeared. CONCLUSIONS:The BMI-related medical expenditures pertained to metabolic disorders, such as hypertension, diabetes, and ischemic heart diseases. A health promotion program should prevent the diseases, thus reducing medical expenditures.