Prevention of systemic fungal infection in the very-low-birthweight infant is important since it is associated with a high morbidity and mortality. To determine if oral nystatin administration could prevent fungal colonization and infection, we evaluated 67 preterm infants with birthweights less than 1250 gm. Thirty-three infants received 1 ml (100,000 units/ml) of nystatin inside the mouth every 8 hours until 1 week after extubation. Oropharyngeal, rectal, blood, and urine cultures were obtained on the 1st day of life and weekly. Endotracheal cultures were obtained three times a week from intubated infants. Four (12%) of the 33 nystatin-treated infants had positive cultures, two (6%) developed systemic infection. The control group consisted of 34 infants, 15 (44%) had positive fungal cultures and 11 (32%) developed systemic infection. Fungi isolated were Candida species and Torulopsis glabrata. Colonized infants were dependent on the respirator (P less than 0.001), had indwelling catheters (P less than 0.01), and received antibiotics (P less than 0.05) for a longer period than infants free from fungi and their mortality was significantly higher (P less than 0.05). We conclude that prophylactic administration of oral nystatin reduces fungal colonization and infection in very-low-birthweight infants.