Objectives: To test and compare the validity of a body mass index (BMI)-based prediction equation and an impedance-based prediction equation for body fat percentage among various European population groups. Design: Cross-sectional observational study. Settings: The study was performed in five different European centres: Maastricht and Wageningen (The Netherlands), Milan and Rome (Italy) and Tampere (Finland), where body composition studies are routinely performed. Subjects: A total of 234 females and 182 males, aged 18-70 y, BMI 17.0-41.9 kg/m2. Methods: The reference method for body fat percentage (BF?F) was either dual-energy X-ray absorptiometry (DXA) or densitometry (underwater weighing). Body fat percentage (BF€was also predicted from BMI, age and sex (BFÆI) or with a hand-held impedance analyser that uses in addition to arm impedance weight, height, age and sex as predictors (BF6P). Results: The overall mean (±s.e.) bias (measured minus predicted) for BFÆI was 0.2±0.3 (NS) and-0.7±0.3 (NS) in females and males, respectively. The bias of BF6P was 0.2±0.2 (NS) and 1.0±0.4 (P