The effects of increasing the dietary polysaccharide content from the customary 40 percent to 50 percent of total energy intake were examined in a metabolic ward-cross-over study of eight noninsulin dependent diabetic patients with normal fasting C-peptide concentration. The aim was to study the effect of a raised carbohydrate content; therefore, the fibre content of the diet was kept approximately constant. Patients had been treated with diet alone, and the high carbohydrate (HC) observation period lasted for two weeks. Blood glucose was significantly increased postprandially in the HC period (11.8 +/- 0.3 versus 10.7 +/- 0.4 mmol/l), and this was accompanied by significantly raised immunoreactive insulin (IRI) concentrations. The secretion of IRI, C-peptide and gastric inhibitory polypeptide (GIP) after a standard meal-challenge at the end of each period were unaffected by the preceding diets. The HC diet was accompanied by increasing values of plasma triglyceride (1.13 +/- 0.06 versus 0.97 +/- 0.06 mmol/l) and VLDL-triglyceride (0.69 +/- 0.04 versus 0.50 +/- 0.04 mmol/l), whereas the ketone body concentration was decreased (0.25 +/- 0.03 versus 0.36 +/- 0.05 mmol/l). Both HDL- and LDL cholesterol were decreased by the HC-diet (1.07 +/- 0.02 versus 1.18 +/- 0.02; 3.45 +/- 0.09 versus 3.89 +/- 0.09 mmol/l, respectively), while the LDL to HDL cholesterol concentration ratio remained unaffected. Thus, in two weeks, a HC diet resulted in hyperglycaemia, hyperinsulinaemia, hypertriglyceridemia and a state of antiketogenesis, without any apparent change in the capacity of mealinduced insulin release. LDL- and HDL-cholesterol were lowered to the same extent.(ABSTRACT TRUNCATED AT 250 WORDS)