OBJECTIVE: To compare high- and low-glycemic index (GI) diets in the treatment of non-insulin-dependent diabetes mellitus (NIDDM). RESEARCH DESIGN AND METHODS: Sixteen subjects with well-controlled NIDDM and normal lipid profile, 10 of whom continued oral hypoglycemic medication, participated in the study. A diet that emphasized low-GI foods (e.g., porridge, pasta) was compared with a high-GI diet (e.g., processed cereals, potatoes). The GI of the low-GI diet was 15% lower than the high-GI diet (77 +/- 3 vs. 91 +/- 1) but otherwise similar in macronutrient composition and fiber, as determined by a 4-day weighed record. The diets were instituted under instruction from a dietitian who visited subjects at home on a weekly basis. Body weight was maintained within 1-2 kg. RESULTS: Glycemic control was improved on the low-GI diet compared with the high-GI diet (statistically significant findings, P less than 0.05). Mean glycosylated hemoglobin at the end of the low-GI diet was 11% lower (7.0 +/- 0.3%) than at the end of the high-GI diet (7.9 +/- 0.5%), and the 8-h plasma glucose profile was lower (area under the curve above fasting 128 +/- 23 vs. 148 +/- 22 mmol.h-1.L-1, respectively). Mean fasting plasma glucose, total cholesterol triglycerides, and lipoproteins did not show important differences. CONCLUSIONS: A low-GI diet gives a modest improvement in long-term glycemic control but not plasma lipids in normolipidemic well-controlled subjects with NIDDM.