OBJECTIVE-The prevalence of gestational diabetes mellitus (GDM) is rising. There is little evidence to demonstrate the effectiveness of one dietary therapy over another. We aimed to investigate the effect of a low-glycemic index (LGI) versus a conventional high-fiber diet on pregnancy outcomes, neonatal anthropometry, and maternal metabolic profile in GDM. RESEARCH DESIGN AND METHODS-Ninety-nine women (age 26-42 years; mean +/- SD prepregnancy BMI 24 +/- 5 kg/m(2)) diagnosed with GDM at 20-32 weeks' gestation were randomized to follow either an LGI (n = 50; target glycemic index [GI] similar to 50) or a high-fiber moderate-GI diet (HF) (n = 49; target GI similar to 60). Dietary intake was assessed by 3-day food records. Pregnancy outcomes were collected from medical records. RESULTS-The LGI group achieved a modestly lower GI than the HF group (mean +/- SEM 47 +/- 1 vs. 53 +/- 1; P < 0.001). At birth, there was no significant difference in birth weight (LGI 3.3 +/- 0.1 kg vs. HF 3.3 +/- 0.1 kg; P = 0.619), birth weight centile (LGI 52.5 +/- 4.3 vs. HF 52.2 +/- 4.0; P = 0.969), prevalence of macrosomia (LGI 2.1% vs. HF 6.7%; P = 0.157), insulin treatment (LGI 53% vs. HF 65%; P = 0.251), or adverse pregnancy outcomes. CONCLUSIONS-In intensively monitored women with GDM, an LGI diet and a conventional HF diet produce similar pregnancy outcomes.