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The influence of age and fitness factors on the validity of prediction equations for estimating percent body fat (%BF) of females was investigated for 60 females, ages 12 to 78 years. Regression equations specific to age and fitness subgroups were derived from a series of skinfold, girth, and diameter measurements of these subjects. The combination of anthropometric variables in these equations providing the highest multiple correlation coefficient varied according to both the aerobic power and age of the subjects. The validity coefficients and standard errors of estimate from previously published equations also varied from group to group, demonstrating again that equations to estimate %BF from anthropometric measurements are specific to the population from which they were derived. As cross-validation of the equation for young adult females in this study demonstrated, prediction errors are of such magnitude that the use of such equations for research or clinical application is highly questionable.