The study had a nested case-control design. In the first year of the project, the diagnosis of BPD was standardized and a scoring system developed for grading the severity of the disease. Baseline data were collected on very low birth weight neonates admitted to seven participating neonatal intensive care units. Maternal interviews were conducted for neonates surviving seven days. The purpose of the interview was to collect information which m;ight be related to a predisposition to BPD, respiratory distress syndrome, or prematurity. Specific areas addressed included familial diseases in general and respiratory diseases in particular, familial patterns of prematurity, pregnancy and birth history, circumstances surrounding delivery, environmental exposures, health habits such as smoking, and socioeconomic variables. Severity scoring and diagnosis of the infants were based on the hospital log and routine x-rays. Final analysis compared infants with BPD to those without BPD, controlling for confounders and placing special emphasis on interaction effects. Full-term neonates served as controls.