To compare outcomes of twin pregnancies with and without gestational diabetes mellitus (GDM). We compared 105 twin pregnancies with GDM (7.8% of all twin pregnancies) to 315 controls without GDM, matched for gestational age, chorionicity and year of birth. Pre-gravid obesity appears to predispose women to GDM during twin pregnancy [odds ratio (OR) 3.5; 95% confidence interval (CI) 1.7, 7.0]. Overweight and obese women that subsequently developed GDM during their twin gestation were less likely to conceive spontaneously (OR 0.4; 95% CI 0.3, 0.7). Twins from the GDM group had more respiratory distress syndrome (RDS, OR 2.2; 95% CI 1.3, 3.7) and had a three-fold, but not significantly increased perinatal mortality rate. Birth weight characteristics were similar in both groups. Twin pregnancies complicated by GDM might be associated with pre-pregnancy maternal obesity and are at increased risk of RDS and non-significant increased risk of perinatal death.