Measurement of milk volume intake (MVI) in infants is a useful tool for assessing nutritional intake. Current measurement techniques include test-weighting, weighting formula-bottles, and calculation of MVI from water turnover estimated isotopically. Test-weighing of breast-fed infants may disturb normal feeding behaviour and thus influence intake. Calculation of MVI from water turnover is a non-invasive measurement method and can be applied to all infants whether breast-fed or formula-fed. The method requires correction for environmental water influx (EWI) which constitutes non-oral water intake. EWI has previously been investigated only in premature or malnourished infants. The correction factor for EWI was calculated by comparing MVI obtained from weighing formula-bottles and measured isotopically in 21 formula-fed infants at 12 weeks of age. The mean correction factor, by which total water intake should be adjusted to obtain oral water intake, was 0.937 (S.E. 0.017). The proportion of water intake derived from non-oral sources was therefore 6.3%.