Objective: To determine the gastrointestinal (GI) tolerance of NUTRIOSE®FB in men. Design: A randomized, placebo-contro lled, parallel, double-blind study. Setting: The metabolic ward of TNO Quality of Life. Subjects: Forty-eight subjects started the study: 16 men participated in one of the three treatments. Subjects consumed either 22.5 g of pure maltodextrin (Glucidex®6), or 30 or 45 g of the dextrin NUTRIOSE®FB daily for 4-5 weeks. Forty-three subjects completed the study (age: 34.7±8.2 years; BMI 24.9±3.3 kg m2). Measurements: Tolerance of NUTRIOSE®FB was examined with a GI complaints questionnaire; effectiveness on colonic flora was examined by faecal analysis; fermentation by breath hydrogen excretion measurement. Furthermore, the effect on body weight (BW), energy intake and blood parameters were examined in the study. Results: Both doses of NUTRIOSE®FB were very well tolerated and GI complaints hardly differed from the placebo treatment. No diarrhoea was reported due to NUTRIOSE®FB supplementation. In the course of the study, some habituation and adaptation of GI symptoms were found. Fermentation and faecal characteristics (pH and enzyme activity) were significantly positively affected with NUTRIOSE®FB treatment. Body weight in both NUTRIOSE®FB groups remained stable over time, although the placebo-treated group showed a small increase in BW (Δday35-1 0.8±1.0 kg) (P=0.07). However, total food intake and macronutrient composition of the diet remained the same throughout the study. No significant differences were found between the three treatment groups in hunger and satiety scores and food preferences. Conclusions: Long-term supplementation of 30 or 45 g of the dextrin NUTRIOSE®FB per day was well tolerated, and may act as a pre-biotic supplement.