Body composition estimates in infants and children are difficult largely due to ethical and practical reasons. However, the use of H218O as a tracer for the calulation of total body water allows the noninvasive estimation of fat-free mass (FFM) and fat mass (FM) in the pediatric population. Body fatness was estimated on 244 occasions in infants and children 0.1 to 3.99 years of age. The mean percentage body fatness in the first year of life is similar to previously published theoretical reference data. During childhood, however, the reference data were considerably below current estimates of body fatness. This may be evidence to support the hypothesis that there has been a secular trend in levels of body fatness in children in recent years. © 1992 Wiley-Liss, Inc.