This study compared body mass index (BMI) and inverted BMI (iBMI) as predictors of body fatness in 177 Portuguese children (149 girls and 96 boys) aged 7-16 years. Participants undertook measures of height and body mass from which BMI (kg/m(2)) and iBMI (cm(2)/kg) were determined. Maturation was determined via self-report and fat mass index (FMI, kg/m(2)) via dual-energy X-ray absorptiometry. Significant relationships were evident between BMI and iBMI and FMI (both P=0.0001). BMI was not normally distributed (P=0.0001) but iBMI was (P>0.05). Analysis of covariance identified that BMI and iBMI, controlling for maturation, were both significant predictors of FMI (both P=0.0001) but that iBMI predicted a slightly greater amount of the variance (adjusted R(2)=0.970) compared with BMI (adjusted R(2)=0.968). This study suggests that iBMI is a similar proxy for body fatness compared with BMI in children.