Bioelectrical impedance (BI) and skinfold thickness (ST) methods are available for indirect determination of human body composition. The validity of BI and ST methods has been established to be sufficiently high in men and women. However, more studies are needed to assess the validity and applicability of these methods to children and youth. This study was designed (1) to investigate the validity of body density measured by BI and ST methods against underwater weighing (UW) as a criterion, and (2) to propose appropriate prediction equations that are applicable to Japanese children and youth. The subjects were 141 boys and 163 girls aged 9 to 14 years. The BI was measured using a tetrapolar bio-impedance analyzer (Selco SIF-891) . The ST was measured at four sites (triceps, subscapular, abdominal, suprailiac) using an Eiken-type caliper. The body density estimated by BI and ST correlated significantly with the hydrodensitometrically determined body density. However, the validity of the BI and ST methods was found to be unsufficient particularly in girls, thereby indicating the need for a more accurate method of assessing body composition in children and youth. We developed multiple regression equations that may be applicable to a wide range of children and youth. We suggest that body composition of younger generation is better esimated from a combination of BI and ST variables.