OBJECTIVE To study the relationship between percent body fat and body mass index (BMI) in two different ethnic groups (Indonesians and Caucasians) in order to evaluate the validity of the BMI cut-off points for obesity. METHODS Cross-sectional study. METHODS Not specially selected populations living in southern Sumatra (Palembang, Indonesia) and Caucasian Dutch living in Wageningen. METHODS Body weight, body height, body fat by deuterium oxide dilution and skinfold thickness. RESULTS Body fat could be well predicted by body mass index (BMI) and sex in the Indonesians and by BMI, sex and age in the Dutch with a prediction error of 3.6 and 3.3% for the two populations respectively. Although the body mass index in the Indonesian group was about 2 kg/m2 lower compared to the Dutch, the amount of body fat was 3% points higher. Because of small differences between the groups in age, weight and height the differences in body fat were corrected for this (ANOVA). Indonesians having the same weight, height, age and sex have generally 4.8% points more body fat compared to Dutch. Indonesians having the same % BF, age and sex have generally a 2.9 kg/m2 lower BMI compared to the Dutch. CONCLUSIONS The results show that the relationship between % BF and BMI is different between Indonesians and Dutch Caucasians. If obesity is regarded as an excess of body fat and not as an excess of weight (increased BMI), the cut-off points for obesity in Indonesia based on the BMI should be 27 kg/m2 instead of 30 kg/m2.