OBJECTIVE To evaluate the influence of gestational diabetes mellitus on neonatal birthweight, macrosomia and weight discrepancy in twin neonates. METHODS An observational retrospective study was performed. One hundred and six women with gestational diabetes and twin pregnancy and 166 twin controls who delivered viable fetuses > 24 weeks were included. Impact of maternal pre-pregnancy BMI, smoking habit, method of conception, chorionicity, gestational age at delivery, mode of delivery and hypertensive complications were also analysed. The effect of maternal hyperglycaemia and metabolic control in gestational diabetes pregnancies was assessed. RESULTS Gestational hypertension and pre-eclampsia were significantly higher in the group with gestational diabetes (21.5% vs. 6.3%, P = 0.007 and 6.2% vs. 0%, P = 0.025). There were no differences in the incidence of macrosomia (5.7% vs. 7.2%, P = 0.803), large for gestational age (10.3% vs. 13.2%, P = 0.570), small for gestational age (10.3% vs. 12.0%, P = 0.701), severely small for gestational age (6.6% vs. 7.8%, P = 0.814) or weight discrepancy (20.6% vs. 15.2%, P = 0.320) in the group with gestational diabetes compared with twin pregnancies without diabetes. There were no differences when comparing insulin-requiring gestational diabetes pregnancies and twins without diabetes for any of the neonatal weight outcomes. There was no relationship between third trimester HbA1c and neonatal birthweight or infant birthweight ratio. CONCLUSIONS Gestational diabetes did not increase the risk of macrosomia or weight discrepancy of twin newborns. Furthermore, glycaemic control did not influence the rate of any of the weight outcomes in our study population. In twin pregnancies, gestational diabetes was associated with a higher risk of gestational hypertension and pre-eclampsia.