OBJECTIVE To evaluate the effect of fibre supplementation in enteral feeding on bowel function in hospitalised geriatric patients, and to assess its metabolic and nutritional efficiency. METHODS Prospective randomised controlled trial with stratification for diabetes. METHODS Department of Geriatrics at the University of Antwerp. METHODS During 30 months (January 2000-June 2002) every hospitalised patient requiring tube feeding was assessed for eligibility (n = 183). Finally 172 patients (19% diabetics) were randomised. METHODS An enteral nutritional regimen consisting of 30 kcal/kg in 2000 ml with a calorie/nitrogen ratio of 156 with or without fibre was instituted. At weekly intervals, stool output was qualitatively evaluated by recording frequency, volume (small <1/2 cup, large >1/2 cup) and consistency (solid-formed, soft-pasty or liquid-watery). Nutritional and metabolic effects were evaluated through laboratory analysis. RESULTS Overall mortality was 24% with a trend for excess mortality in diabetic patients (33.3% versus 21.6% in non-diabetics; P = 0.176). There was no difference in duration of feeding between the fibre group (27.5 days; 95% CI = 19.1-35.9) and the no fibre group (27.9 days; 95% CI = 20.2-35.5). In the fibre-supplemented group, stool frequency was lower (4.1 per week; 95% CI = 3.7-4.6) than in controls (6.3 per week; 95% CI = 5.6-6.9). Qualitatively, stool consistency was higher (P < 0.001) but no difference in volume was noted. There were no differences in final laboratory parameters between groups. CONCLUSIONS Fibre supplementation improved bowel function with reduced stool frequency and more solid stool consistency. It did not affect the nutritional efficiency of enteral feeding in hospitalised geriatric patients. Diabetes may be a risk factor for mortality in malnourished patients requiring tube feeding.