OBJECTIVE: To evaluate in type 1 diabetic patients 1) the long-term feasibility of a high-fiber (HF) diet composed exclusively of natural foodstuffs and 2) the efficacy of this diet in relation to blood glucose control and incidence of hypoglycemic episodes. RESEARCH DESIGN AND METHODS: The study was randomized with parallel groups. Participants were part of a larger multicenter study on the effects of acarbose on glucose control in diabetes. A total of 63 type 1 diabetic patients, age 28 +/- 9 years, BMI 24 +/- 0.6 kg/m2, after a 4-week run-in period on their habitual diet, were randomized to either an HF (n = 32) or a low-fiber (LF) diet (n = 31) for 24 weeks. The two diets, composed exclusively of natural foodstuffs, were weight-maintaining and, aside from their fiber content, were similar for all nutrients. At the end of the run-in period and the dietary treatment, fasting blood samples for the measurement of plasma cholesterol, HDL cholesterol, triglyceride, and HbA(1c) were collected. A daily glycemic profile was performed on a day in which the participants had consumed a standard menu representative of their treatment diet (HF or LF). RESULTS: Of the 63 study subjects, 29 in the HF group (91%) and 25 in the LF group (81%) completed the study Compared with the LF diet, the HF diet after 24 weeks decreased both mean daily blood glucose concentrations (P < 0.05) and number of hypoglycemic events (P < 0.01). When compliance to diet was taken into account, 83% of the subjects on the HF diet and 88% on the LF diet were compliant. In this subgroup, compared with the LF diet, the HF diet significantly reduced mean daily blood glucose concentrations (P < 0.001), HbA(1c) (P < 0.05), and number of hypoglycemic events (P < 0.01). CONCLUSIONS: In type 1 diabetic patients, an HF diet is feasible in the long term and, compared with an LF diet, improves glycemic control and reduces the number of hypoglycemic events.