Metabolic balances were carried out in nine preterm very-low-birth-weight (VLBW) infants, five of whom were fed on expressed breast milk and four on a preterm formula. In 39 stool samples from these infants, there was a very close relationship between the fecal fat and the fecal water (r = -0.95; p less than 0.001), independent of the type of feed. This relationship was present over a very wide range of fat and water contents. Validation experiments confirmed the reproducibility and reliability of this relationship. The correlation is clinically and experimentally so close that measurement of stool water by freeze-drying a sample is an accurate indirect measure of fecal fat in VLBW infants when the daily stool weight was known.