During the first weeks after birth the transepidermal water loss was determined repeatedly in 7 infants born after 25-27 weeks of gestation, in 13 infants born after 28-30 weeks and in 14 full-term infants. All infants were appropriate for gestational age. The very pre-term infants had high evaporative water losses from their skin on their first days after birth. A gradual decrease was then observed, but after four weeks the values were still higher than those in full-term infants. The transepidermal water loss in full-term infants was unchanged during the first two weeks of life, but increased during the subsequent two weeks. The relation between evaporation rate from an interscapular skin area and ambient humidity was studied repeatedly during the first week of life in two very pre-term infants. The magnitude of evaporation decreased with increasing post-natal age, but was still dependent on the ambient relative humidity.