To determine the effect of infant feeding mode on body composition, a cross-sectional study was designed in which 10 breast-fed and 10 formula-fed infants were studied at 1 month of age, and another 10 breast-fed and 10 formula-fed infants at 4 months of age. Anthropometric measurements included body weights, lengths, selected diameters, circumferences and skinfold thicknesses. Total body water (TBWO) was measured by 18O dilution. A dose equivalent to 300 mg 18O/kg body weight was administered orally to the infants. Fat-free mass (FFMO) was calculated from TBWO using reference hydration constants of 0.805 and 0.798 at 1 and 4 months, respectively. Body fatO was taken as the difference between weight and FFMO. Total-body electrical conductivity (TOBEC) measurements were used to estimate FFMT and FATT. ANOVA was used to analyze the anthropometric and body composition data using feeding mode and age as grouping factors. Anthropometric measurements did not differ by feeding mode. TBW (kg) and FFM (kg) and body fat (kg) derived from 18O dilution or TOBEC did not differ by feeding mode. TBWO,T (%wt), FFMO,T (%wt), and body fatO,T (%wt) derived from 18O dilution and TOBEC differed significantly between the breast-fed and formula-fed infants at 4 months of age (p < 0.05). Expressed as a percentage of body weight, TBWO and FFMO,T were higher and body fatO,T was lower among the 4-month formula-fed infants.