Subcutaneous fat mass (SFM) and total fat mass (TFM) were estimated in 16 preterm infants at 8 +/- 2 days postnatal age. SFM was estimated from anthropometric measurements: 5 skinfold thicknesses, 5 body circumferences, and regional body lengths. TFM was estimated from total body water measurements using dilution of isotopic water, H2(18)O. The proportion of TFM that was SFM decreased with an increase in TFM. In infants with less than 100 g of TFM (5% body weight), 74% of TFM is in the subcutaneous layer compared with only 35% in infants with higher TFM (more than 100 g). Subscapular skinfold thickness is the best predictor of TFM (r = 0.78). Additional skinfold thickness measurements (triceps, biceps, supra-iliac and thigh) do not improve the predictability of TFM by the subscapular skinfold thickness. We conclude that measurement of subscapular skinfold thickness is an appropriate choice for estimating fat deposition in preterm infants, particularly in field studies and in developing countries.