To estimate the relationship between the rate of gestational weight gain before the 50-g, 1-hour oral glucose challenge test screening for gestational diabetes mellitus (GDM) and subsequent risk of GDM.We conducted a nested case-control study (345 women with GDM and 800 women in the control group) within a multiethnic cohort of women delivering between 1996 and 1998 who were screened for GDM at 24-28 weeks of gestation. GDM was diagnosed according to the National Diabetes Data Group plasma glucose cut-offs for the 100-g, 3-hour oral glucose tolerance test. Women's plasma glucose levels, weights, and covariate data were obtained by medical record chart review.After adjusting for age at delivery, race/ethnicity, parity, and prepregnancy body mass index, the risk of GDM increased with increasing rates of gestational weight gain. Compared with the lowest tertile of rate of gestational weight gain (less than 0.27 kg/week [less than 0.60 lb/wk]), a rate of weight gain from 0.27-0.40 kg/wk (0.60-0.88 lb/wk) and 0.41 kg/wk (0.89 lb/wk) or more, were associated with increased risks of GDM (odds ratio 1.43, 95% confidence interval 0.96-2.14; and odds ratio 1.74, 95% confidence interval 1.16-2.60, respectively). The association between the rate of gestational weight gain and GDM was primarily attributed to increased weight gain in the first trimester. The association was stronger in overweight [corrected] and nonwhite women.High rates of gestational weight gain, especially early in pregnancy, may increase a woman's risk of GDM. Gestational weight gain during early pregnancy may represent a modifiable risk factor for GDM and needs more attention from health care providers.