The response of blood glucose and serum lipids and lipoproteins to a high-carbohydrate, high-fiber, low-fat diet was assessed in 10 insulin-dependent diabetic subjects. The diet contained approximately 60% of calories as carbohydrate (CHO) and 20% as fat. The patients were followed for 2 wk in a metabolic ward and subsequently for 4 wk at home without changing insulin dosage. During this 6-wk period, the fasting blood glucose fell from 10.6 +/- 1.1 to 8.9 +/- 1.3 mmol/L (NS); HbA1 fell from 11.7 +/- 0.5 to 11.0 +/- 0.7% (P less than 0.05). Serum total triglyceride and very-low-density lipoprotein levels remained unchanged. After 2 wk in the ward on a high-CHO diet, total cholesterol fell by 15% (P less than 0.01), LDL cholesterol by 16% (P less than 0.001), and HDL cholesterol by 10% (P less than 0.05). The fall of HDL cholesterol was due to a decrease of HDL3 cholesterol only. After the 4-wk home period on a high-CHO diet, the observed lipoprotein changes were reversed. Heparin-releasable adipose tissue LPL activity was not influenced by a high-CHO diet. In conclusion, a high-carbohydrate, high-fiber, low-fat diet did not deteriorate the diabetic control, and it had no unfavorable effects on serum lipids or lipoproteins.