To determine whether selenium deficiency is common among low birth weight infants in our neonatal intensive care unit, we surveyed blood samples from healthy full-term and preterm infants born in our hospital over a 3-month period. Selenium was measured by electrothermal atomic absorption spectrometry. Glutathione peroxidase was measured in plasma by an automated method. Baseline (less than 72 hours postnatal) selenium concentration and glutathione peroxidase activity were significantly lower in low birth weight infants than in full-term babies. Sequential selenium analyses were obtained in 16 sick low birth weight neonates who remained in the intensive care nursery for up to 6 weeks because of lung disease. All were fed parenterally without supplemental selenium, with or without oral intake, for periods varying from 3 to 60 days. All had a marked decrease from baseline selenium levels, and values below the detection limit of our assay were found in seven infants. Selenium deficiency is much more common in small infants than is generally realized, but the clinical significance in neonates is poorly understood.