69 measurements of water loss from the skin were made on 57 preterm and newborn babies (gestational age 31-40 weeks; body weight 900 g-350 g) in their first week of life using the Evaporimeter-method. All measurements were made at 18 different skin sites of the naked baby in a standard single-walled incubator. A graphic method described the correlation between transepidermal water loss and body weight. The mean value of water loss in babies weighing greater than 1850 g was 6,6 g/m2h (10,2 ml/kg 24h) and increased up to threefold values when body weights were smaller. A similar course was found during phototherapy: average values of 8 g/m2h (12,7 ml/kg 24 h) were measured in babies weighing greater than 1900 g increasing markedly in lower birth weights. Some single observations are reported.