Twin pregnancies are becoming common as a result of increased assisted reproduction. Studies have shown that the smaller twin of a pair is at greater risk of morbidity and mortality. Our aim was to determine if there is a relation between birth-weight discordancy and neonatal morbidity and mortality and to test the occurrence of respiratory distress syndrome (RDS) in discordant twins. For 5 years all twins admitted to our intensive care unit were included in the study. Discordance was calculated as the difference between twins' weights divided by the heavier weight. Diagnosis of RDS was made by typical clinical and radiographic methods. Multiple logistic regression was performed with gender, weight order and birth order as the independent variables and RDS as the dependent variable. Two hundred and sixty-six twin pairs with a mean gestation of 33 weeks and a mean birth-weight of 1890 g were evaluated. Multiple logistic regression revealed that being the heavier twin was a more important risk factor for RDS (odds ratio 4.5; 95% confidence interval 2.2-9.2) than being the male or second-born twin. Based on neonatal outcomes a birth-weight discordance equal or greater than 20% in twin pairs was accepted as the discordance criterion. Discordant and concordant groups were statistically different in neonatal mortality, necrotizing enterocolitis, polycytemia and hypoglycemia. Our data demonstrated that being the heavier twin is a risk factor for RDS and a birth-weight difference of 20% or more in twin pairs should be accepted as discordance.