OBJECTIVE: To develop equations, from some simple anthropometric measurements, for the prediction of body density from underwater weighing in male spanish children and adolescents. SUBJECTS: One hundred and seventy-five males, aged 7.0-16.9 y, participated in this study, they were recruited from primary and secondary schools. MEASUREMENTS: Body weight and height and skinfold thicknesses by anthropometry, body density by underwater weighing. RESULTS: Correlations between body density and body mass index (BMI) were high until 14.0-16.9y. Correlations between body density and log sigma 4 skinfolds were higher than those with BMI at all ages. Log sigma 4 skinfolds explained between 61% (14.0-16.9 y) and 68% (11.0-13.9 y) of the body density variance. Regression equations for body density from BMI and triceps skinfold thickness explained between 51% (14.0-16.9y) and 68% (7.0-10.9 y) of the body density variance. CONCLUSIONS: The best estimators of body density in the children and adolescents studied were log sigma 4 skinfolds and a combination of BMI and triceps skinfold.