Subjects: Twenty-seven children and adolescents from the Lipid and Obesity Outpatient Clinic, Department of Pediatrics, University of Vienna, were included in the study (14 boys and 13 girls between 6 and 18 y; mean age 12.6 and 13.1 y). Methods: The body fat percentage was measured with BIA (bioelectrical impedance analyzer BIA 2000-M) and DXA (dual energy X-ray absorptiometry) methods on the same day. Results: The mean difference between the body fat mass percentage measured by BIA and DXA was 4.48 with a standard deviation of 2.93. The results measured by BIA were almost always lower than that by DXA by about 12%. The lower and upper limit of the difference in 95% con®dence interval was À5.64 and À3.32. By paired t-test, these results were signi®cantly different (P`0.001). The correlation between the two measurements was 0.826. The mean percentage of body fat mass measured by BIA was 34.86 AE 7.08% and by DXA 39.75 AE 5.63%. The differences were not changed by age and body fat percentage but they were by sex. Conclusions:The results of the study show that the body fat percentages measured by BIA and DXA method were signi®cantly different. This is very important because BIA technique is a routine technique for clinical purposes. Adjustments to the formula used for calculating the total fat mass in obese children and adolescence are necessary. Underestimation of body fat percentage measured by bioelectrical impedance analysis compared to dual X-ray absorptiometry method in obese children is three times higher with boys than with girls. Sponsorship: