OBJECTIVE: To examine the relative effects of high and low glycemic index (GI) carbohydrates, and monounsaturated fats on blood glucose and lipid metabolism in NIDDM subjects. SUBJECTS: Fourteen male and seven female variably controlled NIDDM subjects recruited by advertisement. SETTING: Free living outpatients. RESEARCH DESIGN: A repeated measures, within-subject design was used such that each subject consumed three diets: (a) a high-GI diet (53% CHO -21% fat, 63 GI units (glucose= 100)); (b) a low-GI diet (51% CHO -23% fat, 43 GI units); and (c) a high-mono high-GI diet (42% CHO -35% fat, 59 GI units) in random order and cross-over fashion for four weeks. Approximately 45% energy was provided as key foods which differed in published GI values and specifically excluded legumes. Dietary fibre intake was > 30 g/d on each diet. At the end of each dietary intervention, we measured fasting plasma lipids, glucose, insulin, total glycated plasma protein, fructosamine, LDL and HDL particle size as well as 24 h urinary excretion of glucose and C-peptide. RESULTS: HDL-cholesterol was higher on the low-GI and high-mono high-GI diets compared to the high-GI diet (P < 0.05 for overall diet effect). There were no other significant differences in metabolic control between diets, even when adjusted for BMI, glucose control or gender. Body weight and saturated fat intake remained stable between dietary interventions. CONCLUSION: High-mono high-GI and high-CHO, low-GI diets are superior to high-CHO, high-GI diets with respect to HDL metabolism but no effect was noted on glucose metabolism in variably controlled NIDDM subjects.