Objective. Previous studies show that body mass index (BMI) does not fully explain differences in percent body fat (%BF) between ethnic groups and few studies have investigated this in adolescents. We sought to compare %BF for a given BMI between adolescents from four ethnic groups and to explain ethnic differences in this relationship. Methods. Weight, height and waist circumference were measured in 202 boys and 197 girls (age range 12–19 years; 129 Pacific Island, 91 European, 90 Maori and 89 Asian Indian). Fat mass, appendicular skeletal muscle mass (ASMM), leg length, bone mineral content (BMC), and fat distribution measures were derived from dual-energy X-ray absorptiometry. Results. For the same BMI and age, compared with European boys, %BF in Maori, Pacific Island and Asian Indian boys was 2.8% lower (P=0.017), 5.2% lower (P<0.0001), and 3.5% higher (P=0.0025), respectively. Compared with European girls, %BF, adjusted for BMI, for Maori, Pacific Island and Asian Indian girls was 1.9% lower (P=0.024), 4.1% lower (P<0.0001) and 3.6% higher (P<0.0001), respectively. Adjustment for ASMM, BMC and fat distribution variables, in particular, significantly reduced the differences between ethnic groups. In boys, readily measured variables, conicity index and waist circumference/height, had notable effects on ethnic differences in %BF. Conclusions. Our results show that BMI is not an equivalent measure of %BF between adolescent Europeans, Maori, Pacific Islanders and Asian Indians. Differences in muscularity, bone mass, relative leg length, fat distribution and body shape contribute to this disparity.