In a randomized, double-blind, controlled clinical study, the effects of 5 g of guar gum, a dietary fiber composed of galactose and mannose, or placebo added to the diet of 20 patients with duodenal ulcer for 1 wk each were examined. Ten patients derived evident benefit and five some help from guar gum, on comparing symptoms during administration of guar gum with those experienced earlier or during the placebo week, whereas four patients found that neither guar gum nor placebo had any effect (p less than 0.001). The beneficial effect was associated with increased feelings of repletion after meals. Patients with fewest symptoms benefited only slightly, or not at all, from guar gum. In one patient, guar gum abolished pain felt earlier and on placebo, but also caused severe gastric retention after meals. This patient had pyloric stenosis. In patients who were intolerant to berries, fruits, sugar, sweet rolls, and pizza these foodstuffs were better tolerated during guar gum administration. The diarrhea which occurs in some patients ingesting guar gum was avoided by giving low initial doses. In three patients unpalatability of guar gum was a minor complaint. It is concluded that guar gum is helpful to many patients with uncomplicated duodenal ulcer, but that it is harmful to those having increased gastric emptying, eg, pyloric stenosis patients, and that guar gum may exert its effects by increasing gastric emptying time.