Dietary fiber and fiber supplements are reviewed, with particular emphasis on their sources, composition and properties; physiological actions on gastrointestinal functions; and uses in gastrointestinal disease states (functional bowel disease, diverticular disease and other conditions). Adverse effects and contraindications, and the hypothesis of diet's effect on colon cancer also are discussed. Dietary fiber supplements may relieve symptoms of constipation, spastic colon, and diverticular disease; in the two latter disorders, colonic pressure relationships are altered. It is concluded that current evidence does not support other therapeutic uses for dietary fiber sonstituents, except possibly in patients with anal fissures and hemorrhoids, which can be helped by the passage of a softer stool.