Thirteen poorly controlled noninsulin-dependent diabetic subjects ingested in succession over 5.5 wk their usual low-carbohydrate, low-fiber diet (LCF) for 3 d, a high-carbohydrate, high-fiber diet (HCF) for 3 wk, and the LCF diet again for 2 wk. All diets were designed to be individually isoenergetic. Fasting plasma glucose fell significantly during the HCF diet and then rose significantly during the last LCF diet. Dietary change rather than hospitalization had its full effect by 18 d. Urinary glucose excretion rose transiently on the HCF diet before also falling significantly. Similarly, pancreatic immunoreactive glucagon (IRG) fell significantly on the HCF diet and increased significantly on the LCF diet. No significant differences were observed in plasma insulin, serum free fatty acids, or monocyte insulin binding activity between the two diets. Reduction in circulating IRG may in part explain the lower fasting (or basal) plasma glucose observed on HCF diets.