Objective-The goal was to determine the magnitude of genetic effects on susceptibility and risk factors for bronchopulmonary dysplasia by using the clinically validated National Institutes of Health consensus definition as a demonstrated proxy for long-term respiratory and neurodevelopmental outcomes in extremely low birth weight infants.Methods-We analyzed clinical data from twin pairs born at ≤30 completed weeks gestation in British Columbia, Canada, between 1993 and 2006. Differences in correlations between monozygotic (MZ) and dizygotic (DZ) twin pairs and model-fitting approaches were used to quantify the relative contribution of genetic, shared environmental and non-shared environmental effects.Results-Among 318 twins of known zygosity, monozygotic twin pair similarities were greater than those observed for dizygotic pairs, which suggest significant heritability for bronchopulmonary dysplasia. Model-fitting analyses confirmed that genetic effects accounted for 82% and 79% of the observed variance in bronchopulmonary dysplasia susceptibility, defined on the basis of the need for supplemental oxygen at 36 weeks or the National Institutes of Health consensus definition, respectively. Variations in rates of hemodynamically significant PDA were largely accounted for by genetic effects, whereas variance in susceptibility to blood-borne bacterial infections was largely attributable environmental factors both common and unique to each infant.Conclusions-Susceptibility to BPD and persistence of PDA are both significantly heritable. Our study strengthens the case for investigating further genetic risk stratification markers useful for predicting the most significant long-term respiratory and neurodevelopmental consequences of bronchopulmonary dysplasia in premature neonates.