A prospective population-based study of chronic lung disease among all very low birthweight infants (birthweight 500-1499 g) born in New Zealand in 1986 is reported. Of 413 of these infants admitted to neonatal units, 355 (86%) survived to 28 days. An additional 50 infants were recorded as liveborn but died in the labour ward or other place of birth. Both observed survival and survival adjusted for birthweight, gestation and gender were significantly (P less than 0.05) better in larger centres. Oxygen requirement was assessed at 28 days of age, 36 weeks equivalent gestation and 84 days of age, when 38.6, 23.1 and 13.8% of infants, respectively, were being treated with oxygen. To examine the joint effects of predictor variables on oxygen requirement at each age, the data were analysed using multiple logistic regression methods. At 28 days, lower birthweight, shorter gestation, respiratory distress syndrome (all P less than 0.0001), and gender and hospital principally caring for the infant (both P less than 0.05) were significantly associated with treatment with oxygen. In comparison with other studies, New Zealand appears to have a relatively high rate of chronic lung disease. We speculate that a contributing factor may be the small size of some regional neonatal units.