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The importance of nutrition in the growth of newborn infants has increased in recent years due to the number of low-birth-weight and significantly ill infants who survive the neonatal period. These infants receive a variety of chemically defined and specialized diets as part of their support, and appropriate methods are needed to measure their growth and nutritional status. Few reports are available, however, on the composition of infant growth (fat versus fat-free mass). The paucity of knowledge is due to the intrinsic difficulties of applying available techniques to newborn infants. The TOBEC (total body electrical conductivity) instrument has been introduced as a rapid, safe, and noninvasive method to determine fat- free mass (FFM) in infants. The instrument operates on the principle that an organism placed in an electromagnetic field, perturbs the field to a degree that depends upon the amount and volume of distribution of its electrolytes. The body composition of 16 infants (age, 2 days to 9.7 months; weight, 2 to 8.7 kg) was measured by the TOBEC method. Total body water (TBW) was determined by the isotope dilution technique using oxygen-18-labeled water. Good correlation was found between the log of the TOBEC signal and TBW (r = 0.949). FFM was calculated from TBW (FFM = TBW/0.82) and by TOBEC using standards derived from mature rabbits and infant miniature pigs. FFM calculated from TBW using the factor 0.82 appeared to be overestimated and was greater than total body weight in five infants. Measured by TOBEC using the adult rabbit standard, FFM ranged from 61 to 91% of total body weight. The infant miniature pig standard curve resulted in an overestimation of FFM as measured by TOBEC. Correcting for the excess extracellular fluid known to be present in infants under 60 days of age resulted in expected values. We conclude that the TOBEC method is highly suitable for use with human infants and determines body composition with greater facility than other available methods.