OBJECTIVE: To describe ethnic differences in the relationship between body size and body composition. Knowledge about such differences is important when studying obesity-related complications, such as hypertension and non-insulin dependent diabetes, because it may not be possible to generalize results from one study population to other populations. DESIGN: Cross-sectional study. SUBJECTS: Four groups of different ethnic identity (2987 Caucasians (Danes), 243 predominantly Melanesian (Torres Strait Islanders from northern Australia), 206 Australian Aborigines and 146 Polynesians (New Zealand Samoans), aged 30-70 y, were studied. MEASUREMENTS: We examined associations between body weight and bioelectrical impedance, as a measure of body composition. RESULTS: Except for Australian Aborigines, associations (slopes) between body weight and resistance were generally constant in the different ethnic groups, once height and age differences had been considered, indicating that this relationship may involve a certain universality, that is independent of the population specificity for impedance measurement. Systematic differences in instrument readings or electrodes did not seem to be responsible for the differences found. CONCLUSION: With the exception of Australian Aborigines, there may be a constant relation between body size and body composition (total body water or fat free mass) of different ethnic groups, that depends on gender and age category only.