Frequency and perinatal risk factors in bronchopulmonary dysplasia (BPD) were retrospectively evaluated in a cohort of 242 infants with birth weights less than 1501 g born in one hospital in 1990-1994. At 28 days' postnatal age, 30.7% (59/192) of the infants alive received oxygen supplementation and showed typical radiological changes in chest X-rays. At 36 weeks' corrected gestation, 13.0% (24/184) of the survivors fulfilled these criteria. In multivariate analysis, low birth weight and gestational age, male sex, packed red cell infusions and long duration of ventilator therapy were correlated with an increased risk of BPD at 28 days' postnatal age. Only 49% of the infants with BPD had had respiratory distress syndrome, and 49% of them recovered from BPD by 36 weeks' corrected gestational age. Preeclampsia, low birth weight, rapid birth weight recovery, packed red cell infusions, long duration of ventilator therapy, patent ductus arteriosus and hyperoxia were associated with BPD beyond 36 weeks' corrected gestation. No infant born small for gestational age recovered from BPD before 36 weeks' corrected gestation. The frequency of BPD at 28 days' postnatal age seems to be increasing, but half of the patients recover before term. Factors other than respiratory distress syndrome, especially small birth weight, early weight gain and possibly intrauterine growth retardation are becoming more important risk factors of BPD beyond 36 weeks' corrected gestation.