There is a dearth of body composition techniques suitable for use in young infants. We have investigated the ability of Quetelet's index (W/H2) to predict body fatness in a group of male and female infants at 5 and 11 weeks of age. Fat-free mass and hence fat mass was determined using an H218O dilution technique. FM/H2 and Quetelet's index were significantly correlated in all but the male, 11 week old infants. However Quetelet's index could not be used in a predictive capacity because of the poor variation accounted for by the relationships. It is possible that Quetelet's index is a poor method of predicting body fatness in young infants because of the very rapid changes that are occurring in body weight and length at that time.