Body fat may be estimated from the body mass index by formulae derived by Deurenberg and co-workers. When making calculations, the relationship between densitometrically determined body fat percentage (BF%) and BMI must take age and sex into account. Internal and external cross-validation of the prediction formulas showed that they gave valid estimates of body fat in males and females at all ages. In obese subjects, however, the prediction formulas slightly overestimated the BF%. The prediction error is comparable to the prediction error obtained with other methods of estimating BF%, such as skinfold thickness measurements and bioelectrical impedance. The formula for children is different; the relationship between BMI and BF% in children was found to differ from that in adults due to the height-related increase in BMI in children aged 15 years and younger.