BACKGROUND: Idiopathic reactive hypoglycaemia (IRH) is a condition characterized by aggravated postprandial glucose excursions in otherwise healthy individuals. We investigated its prevalence and the impact of fibre diet supplementation. METHODS: First, IRH prevalence was assessed in 362 subjects without a diagnosis of abnormal glucose metabolism through an oral glucose tolerance test (OGTT). IRH was defined by 1 h- or 2 h-glucose ≤3.9 mmol/L or 1 h- or 2 h-glucose < fasting glucose. Second, in a cross-over trial we evaluated effects of 2 weeks with, and without, 20 g fibre (fructose- oligosaccharides) diet supplementation in subjects with IRH. At the end of each 2-week cycle we analysed fasting biomarker levels and conducted a 4 h-OGTT. RESULTS: IRH was found in 12.4% and a normal glucose tolerance in 56.4% of the participants. The IRH group was characterized by higher fasting (5.3 vs. 5.2 mmol/L, p < 0.05) but lower 2 h- (4.4 vs. 6.5 mmol/L, p < 0.01) glucose levels, whereas age (68 ± 10 vs. 70 ± 9 years) and BMI (24.7 ± 3.3 vs 25.0 ± 3.5 kg/m(2)) were similar. The 2-week fibre diet-supplementation (n = 12, age 56 ± 8 years, 6 females, BMI 25.0 ± 2.9 kg/m(2)) improved both the reactive glucose pattern during the 4 h-OGTT (significantly increased late-onset glucose nadirs and reduced the frequency of glucose ≤3.9 mmol/L [21 to 11, p = 0.04]) and reduced fasting plasma glucose (5.4 ± 0.6 to 5.1 ± 0.5 [p < 0.05]) and total cholesterol (5.3 ± 1.1 to 4.9 ± 1.1 mmol/L [p < 0.04]). CONCLUSIONS: A reactive glucose pattern following intake of a high glycaemic load is relatively prevalent and this phenomenon could be modulated by dietary fibre supplementation.