Fifteen non-insulin-dependent diabetic patients with persistently elevated blood glucoses despite high doses of oral hypoglycaemic agents, were randomly allocated to a high carbohydrate-high fibre diet (HC) or a reinforced low carbohydrate diet (LC). After six weeks the diets were reversed for a similar period. Immediately preceding the study and at the end of each dietary period 24-h biochemical profiles were performed. In the 11 patients who completed the study, fasting and preprandial glucose, percentage glycosylated haemoglobin, VLDL cholesterol and mean 24-h triglycerides were significantly lower on HC than on LC or during the initial profile on their usual diet. There was no significant difference in any of the measurements on LC compared with the usual diet. Previous studies of high carbohydrate-high fibre diets in diabetes have been carried out in relatively well-controlled patients. These data show that poorly controlled non-insulin-dependent patients have an even more striking response.