OBJECTIVE: The objective of the study was to evaluate obesity and rate of weight change during the 5 years before pregnancy and risk of gestational diabetes mellitus (GDM) in a nested case-control study. STUDY DESIGN: GDM cases (n = 251) and controls (n = 204) were selected from a multiethnic cohort of 14,235 women who delivered a live birth between 1996 and 1998. Women who gained or lost weight were compared with those with a stable weight (+/- 1.0 kg/year). RESULTS: Women who gained weight at a rate of 1.1 to 2.2 kg/year had a small increased risk of GDM (odds ratio [OR] 1.63 [95% confidence interval (CI) 0.95 to 2.81]) and women who gained weight at a rate of 2.3 to 10.0 kg/year had a 2.5-fold increased risk of GDM (OR 2.61 [95% CI, 1.50 to 4.57]), compared with women with stable weight (after adjusting for age, race-ethnicity, parity, and baseline body mass index). CONCLUSION: Weight gain in the 5 years before pregnancy may increase the risk of GDM.