Objective: The purpose of this study was to compare pregnancy outcomes in women with diet-treated gestational diabetes mellitus (GDM) that was diagnosed at <24 weeks of gestation to those women who received the diagnosis at ≥24 weeks of gestation. Study Design: This was a retrospective cohort study of 2596 women with diet-treated GDM who delivered between December 1999 and June 2005 at Parkland Hospital. Women with risk factors for GDM underwent immediate glucose screening; women without risk factors underwent universal glucose screening between 24 and 28 weeks of gestation. Women with diet-treated GDM that was diagnosed at <24 weeks of gestation (n = 339; 13.1%) were compared with those women who received the diagnosis at ≥24 weeks of gestation. Results: Women with an earlier diagnosis of diet-treated GDM were at increased risk of preeclampsia and the delivery of large infants. Even after adjustment for differences in maternal characteristics and glycemic control, the risk of preeclampsia persisted (odds ratio, 2.4; 95% CI, 1.5, 3.8). Conclusion: Women with an early diagnosis of diet-treated GDM have a 2-fold increased risk of preeclampsia. © 2008 Mosby, Inc. All rights reserved.