A retrospective survey was carried out in a neonatal unit to identify babies who required oxygen for more than the first 28 days of life and to determine the cause of their oxygen dependency. A total of 9173 neonates were admitted over a three year period. Approximately 750 were ventilated. Fifteen required oxygen for more than the first 28 days. Ten (67%) of these were due to bronchopulmonary dysplasia. 2 had upper airway complications of mechanical ventilation, one had recurrent apnoea, one had recurrent pneumonia, and one who did not require ventilation had chronic oxygen dependency of unknown cause. Bronchopulmonary dysplasia was the commonest cause of chronic oxygen dependency. The incidence was 1.5% of ventilated babies.