Context: Women who have diabetes mellitus during pregnancy are at higher risk of adverse outcomes. Excessive gestational weight gain (GWG) is also emerging as a risk factor for maternofetal complications, and in 2009, the Institute of Medicine published recommendations for appropriate GWG. It is unclear whether excessive GWG confers additional risk to women with diabetes in pregnancy and whether Institute of Medicine recommendations are applicable to this population.Objective: The objective of this study was to examine whether excessive GWG in pregnancies complicated by diabetes mellitus is associated with higher adverse obstetric outcomes.Design: This was an observational study.Setting: The study was conducted at five antenatal centers along the Irish Atlantic seaboard.Participants: 802 women with diabetes in pregnancy participated in the study.Main Outcome Measure: Maternal outcomes examined included preeclampsia, gestational hypertension, and cesarean delivery. Fetal outcomes included large for gestational age (LGA), macrosomia, and small for gestational age.Results: Excessive GWG was noted in 59% of women. In all women, excessive GWG resulted in higher odds for LGA [adjusted odds ratio (aOR) 2.01, 95% confidence intervals 1.24-3.25 in GDM; aOR 3.97, CI 1.85-8.53 in pregestational diabetes mellitus (PGDM)] and macrosomia (aOR 2.17, CI 1.32-3.55 in GDM; aOR 3.58, CI 1.77-7.24 in PGDM). Excessive GWG was also associated with an increased odds for gestational hypertension (aOR 1.72, CI 1.04-2.85) in women with GDM, and treatment with insulin further increased the odds for LGA (aOR 2.80, CI 1.23-6.38) and macrosomia (aOR 5.63, CI 2.16-14.69) in this group.Conclusion: We show that in the already high-risk settings of both GDM and PGDM, excessive GWG confers an additive risk for LGA birth weight, macrosomia, and gestational hypertension.