A prospective study on the incidence of bronchopulmonary dysplasia (BPD) in premature infants is reported. A cohort of premature infants with gestational ages < or = 32 weeks, treated during 1989, was followed for one year. Of a total study population of 117 infants, 23 (19.6%) developed BPD, defined as oxygen dependence at 28 postnatal days. However, only 15 infants (12.8%) needed supplementary oxygen at the age of 36 gestational weeks and 5 infants (4.2%) needed supplementary oxygen periodically at one year of age. BPD was found to account for a significant part of both the total and late mortality in the cohort. Measurements of pulmonary mechanics were performed at 3 +/- 1 and 12(13) +/- 1 days of life in a subgroup of 26 infants with RDS who required assisted ventilation for 4 days or longer. No significant difference in lung compliance or resistance could be found during the first examination between infants who later developed BPD and infants with RDS only. At the second examination, infants who later developed BPD had significantly lower lung compliance (0.48 +/- 0.23 ml/cmH2O) than infants in the RDS group (1.50 +/- 0.72 ml/cmH2O) (p < 0.001). Measurements of pulmonary mechanics could be of importance for early prediction of infants at risk of BPD.