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-ninewomen (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] ∼50) or a high-fiber moderate-GI diet (HF) (n = 49; target GI ∼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.