The effects of an increase in dietary fiber include: (a) More frequent stools, (b) more voluminous stools, (c) an alteration in the fecal flora, (d) an increase in fecal sterol excretion, and, it appears likely, (e) a reduction in intraluminal pressures in the sigmoid colon. Epidemiologic data comparing global differences in prevalence of certain diseases with the fiber content of diets suggest that there may be a relationship between the two. With a certain amount of post-ad hoc reasoning, it can be shown that some of the known effects of fiber could account for differences in disease prevalence between populations. The prevalence of irritable bowel syndrome is so high that one is forced to concede the very real possibility that the environment, including the diet, may be responsible for symptoms that might not otherwise exist. It remains to be seen whether a marked increase in dietary fiber will prevent the symptoms of irritable bowel syndrome. It seems fairly certain that, given the preoccupation most of these individuals have with their bowel movements, the large bulky stools resulting from a high-fiber diet satisfy a very basic emotional need to "have a good BM" (meaning large), but whether this leads to better health remains to be proved.