Diarrhea is a common complication of enteral feeding in critically ill patients. This placebo-controlled clinical trial assessed the effect of adding fiber in the form of 'Fybogel' (Ispaghula husk), one sachet twice daily, on the occurrence of diarrhea during enteral feeding. Sixty-eight patients without prospectively defined exclusion criteria were enterally fed with 'Osmolite' in the Intensive Care Unit during the study period, 35 receiving 'Fybogel', and 33 placebo. Nineteen patients in each group had diarrhea on at least 1 day during enteral feeding, with 66 (23%) feeding days complicated by diarrhea in the 'Fybogel' group, and 68 (23%) in the placebo group. Narcotic infusions, thiopentone infusions, 'Mylanta', H2-antagonists, and nystatin suspension did not significantly affect the incidence of diarrhea. Weak correlations were found between diarrhea and the number of antibiotics each patient received (r = 0.2, p less than 0.05) and also the number of positive nonenteral bacterial cultures (r = 0.2, p less than 0.05). The addition of fiber in the form of 'Fybogel' to enteral feeds did not affect the occurrence of diarrhea.