Assessing body composition during infancy requires data for the so-called reference infant. Currently available data for this purpose need to be updated and extended using methods based on principles different from those used previously to define the reference infant. Thus, magnetic resonance imaging was applied to full-term healthy boys (n = 25) and girls (n = 21), 4-131 d old, to estimate adipose tissue volume (ATV) and the amounts of s.c. and non-s.c. adipose tissue (AT). Total body water was estimated using isotope dilution. Total body fat (TBF), fat free weight (FFW) and the degree of hydration in FFW were calculated. Increases in weight, TBF, and FFW with age agreed with current reference data, although when compared with the reference, a slightly more rapid increase in % TBF was observed for boys. The degree of hydration in FFW was 78.9 +/- 4.5% (n = 45). Both sexes showed significant increases with age in s.c. ATV (14.7 and 13.0 mL/d for boys and girls, respectively) and in non-s.c. ATV (1.58 and 1.26 mL/d, respectively). Subcutaneous ATV was 90.5 +/- 1.8% (boys) and 91.1 +/- 1.9% (girls) of total ATV. In conclusion, a pronounced increase with age in the amount of AT was demonstrated involving a considerable gain in s.c. fat during early life. Except for % TBF in boys, changes in body composition with age agreed with current reference data.