We investigated the selenium status of 5 infants while on nutritional support. After 4 weeks of parenteral nutrition a significant fall in plasma selenium concentrations was observed (mean +/- SD: 0.8 +/- 0.5 micrograms/dl; normal for this age: 3.6 +/- 0.9 micrograms/dl). In 1 infant the decline in selenium value occurred simultaneously with a transient rise in transaminases. A parallel but delayed decrease in red blood cell-glutathione peroxidase activity was seen in 3 patients. After reintroduction of enteral foods, the selenium levels increased progressively to and reached control values after 6 weeks, 4 and 5 months respectively in 3 patients, suggesting that the selenium requirement on TPN was not met. We consider it essential to provide longterm TPN patients with physiological amounts of selenium in order to prevent the progressive development of a deficiency state.