Examine outcomes among birth weight concordant and discordant 401-1500 g twins.Twins (n = 8,114) at NICHD Neonatal Research Network (1994-2011) were studied. Discordance (birth weight difference/larger twin birth weight x 100%) was categorized into: ≤ 14, > 14-20, > 20-30, and > 30%. Separate logistic regression models for the smaller and larger infants assessed the adjusted association between discordance and outcomes.Compared with the smaller twin with ≤ 14% discordance, mortality, necrotizing enterocolitis, severe retinopathy of prematurity, bronchopulmonary dysplasia, and neurodevelopmental impairment or death were highest among the smaller twins with discordance > 30%. The larger twins with discordance > 30% had higher odds of patent ductus arteriosus, moderate-to-severe cerebral palsy, blindness, cognitive and motor scores < 70. Odds of cerebral palsy and blindness were also higher among the larger twins with discordance > 14-20%.Discordance > 30% was associated with higher mortality in the smaller twin and higher morbidities among the smaller and larger twins.