OBJECTIVE To assess the effect of the combination of gestational diabetes mellitus (GDM) and pre-gravid obesity ('diabesity') in singleton gestations. METHODS We compared perinatal outcomes of singleton gestations in mothers with GDM and pre-gravid obesity, with GDM but with normal pre-gravid BMI, and obese mothers without GDM. RESULTS We compared diabesity mothers (n = 1525, 24.4% of mothers with GDM, 9.9% of all obese women) to mothers with GDM but with normal BMI (n = 4704, 75.6% of mothers with GDM) and to obese mothers without GDM (n = 13,937, 90.1% of all obese mothers). Obesity, with and without GDM, increased the odds of having chronic hypertension whereas preeclampsia appears to be influenced by obesity only, as were the risk of births at <33 weeks' gestation, of birth weight >4000 g, low 5-min Apgar scores and NICU admissions. CONCLUSION Obesity (without diabetes) is more frequently associated with adverse perinatal outcomes than diabesity or GDM in non-obese mothers. A campaign to decrease pre-gravid obesity should have at least the same priority as any campaign to control GDM.