The whole body counter has provided a fast and accurate method of determining the total body potassium (TBK) of an individual. The interpretation of a single determination does present difficulties because of individual differences in body composition. The normal range, expressed in milliequivalents per kilogram, is so wide that an individual can lose 33% of his TBK and the result be interpreted as normal. To overcome this problem the prediction of the normal TBK of an individual from functions of weight (W), height (H), and skinfold thickness has been investigated and the accuracy of prediction of TBK and log TBK by various functions of weight, height, and skinfold thickness compared. Eighty-seven children aged1.5-27 months and of varying height and weight for age were studied. None had any abnormality which may have resulted in potassium depletion at the time of study. The best prediction of TBK (in milliequivalents or milliequivalents per kilogram) by functions of weight and or height f(W, H) had 95% tolerance limits of 100% ± 18.2%. Using a function of the log transformation of mean skinfold thickness including a f(W, H) narrowed the tolerance limits to ± 6%. The best prediction of the log TBK (in milliequivalents or milliequiva-lents per kilogram) by f(W, H) gave tolerance limits of ±3.3%-4.8%. The best prediction of the log TBK in milli-equivalents was f(W, H) and log transformation of mean skinfold thickness which gave narrow 95% tolerance limits of 100 ± 1.3%. © 1974 International Pediatric Research Foundation, Inc.