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The relationships among ten skinfold thickness measurements, body weight, and lean body mass, estimated from whole-body 4,,K measurement, in 162 boys ranging in age from 6.3 to 12.9 years were investigated over a three year period. With 75.5% of the variation in lean body mass associated with body weight alone (r = .87) lean body mass was predicted from body weight with a standard error of estimate of 3.0 kg ( 10.9% of the mean). The error of prediction was significantly greater for children heavier than 40 kg than for children lighter than 30 kg. Body weight and two skinfold measures, upper arm and back, were found to be the variables contributing most to the prediction of lean body mass using a step-down multiple regression procedure. Together they accounted for 91.3% of the variation (R = .96) in lean body mass with a standard error of estimate of 1.8 kg. The other eight skinfold measures, height and chronological age contributed little to the prediction. Similar results were found within each year of the investigation, within each of the five age groups and within each of four weight groups. Racial background of the boys was not a significant factor in the prediction of lean body mass. The multiple regression equation based on two skinfolds and body weight was tested on an independent sample of boys and its applicability for predicting lean body mass in six to twelve-year-old boys substantiated.