Nine male and seven female high level ballet students had total body water (TBW) measured by two methods: 1) H2(18)O isotope dilution and 2) total body electrical conductivity (TOBEC). TBW from TOBEC measurements was calculated using an equation previously derived from adults, and these new measurements correlated highly with TBW by isotope dilution (r = 0.99, SEE = 0.78 kg). However, the difference between these two methods increased as TBW increased. Therefore, a new equation (ballet equation) was developed which improved the prediction of TBW for individuals (SEE = 0.69 kg) and for which the difference in TBW between isotope dilution and TOBEC was independent of the TBW value. Using age-adjusted hydration constants, the TBW measurements by isotope dilution were converted to fat-free mass, and these values were used to develop an accurate equation for predicting fat-free mass from TOBEC measurements (r = 0.99, SEE = 0.91 kg). In addition, we measured TBW and calculated FFM in ten control adolescents and used these values to develop equations for predicting TBW (r = 0.99, SEE = 1.01 kg) and FFM (r = 0.99, SEE = 1.42 kg) in a combined group of adolescent dancers and non-dancers. Total body water and fat-free mass can be accurately measured in adolescent ballet dancers and non-dancers using TOBEC.