To evaluate the influence of a low glycaemic index (GI), high GI and high fibre diet on glycaemic control and insulin requirement in Type 1 diabetic patients on intensive insulin therapy, nine well-controlled, highly-motivated Type 1 diabetic patients were put on a control diet for 12 days and then randomized in a consecutive manner to 12 days of each diet, in a crossover design. During each experimental diet, the study subjects adjusted their premeal insulin (soluble) dose to maintain their 1-h postprandial capillary glucose at or below 10 mmol l(-1). At the end of each experimental diet, they were submitted to a standardized breakfast of the diet under study, using the same premeal insulin dose as that required for the control diet. The control diet contained 16.0+/-3.0 g of fibre day(-1) with a GI of 77.4+/-2.7 compared to 15.3+/-6.3 and 66.2+/-1.2 for the low GI diet, 17.1+/-7.2 and 92.9+/-3.6 for the high GI diet, and 56.1+/-3.6 (including 15 g of guar) and 73.5+/-2.1 for the high fibre diet. Prebreakfast capillary blood glucose (6.2+/-1.2 mmol l(-1)) on the low GI diet and postbreakfast capillary blood glucose (8.7+/-1.8 mmol l(-1)) on the high fibre diet were significantly lower than the values obtained with the control diet (8.0+/-1.8 and 10.6+/-2.4, respectively; p<0.05). No change in premeal or basal insulin dose was required. During the standardized breakfasts, the incremental area under the curve was 1.6+/-1.5 mmol l(-1) min(-1) for the control diet compared to 1.1+/-1.8 for the low GI diet, 3.2+/-1.4 for the high GI diet (p<0.05 versus low GI and high fibre; p=0.08 versus control), and 1.0+/-0.9 for the high fibre diet. These observations indicate that in well-controlled Type 1 diabetic subjects on intensive insulin therapy, major alterations in the GI and fibre content of meals induce small but significant changes in glucose profile. In everyday life, however, these differences are blunted, and plasma glucose remains within the target range for optimal metabolic control.