Introduction Diarrhea is defined as excess stool water, usually greater than 200 gm/day (or greater than 10 gmikglday in infants). Frequency per se, is not indicative of diarrhea, although they often coexist. Indeed, breast-fed infants may have up to 12 stools/day, and formula-fed infants, up to seven stools/day. The presence of undigested food particles merely indicates fast transit. Acute infective diarrhea is usually an obvious increase over the baseline with nausea, vomiting, fever, and abdominal pain variably present. Although most of the episodes in children are self-limited, diarrhea continues to be a major global problem, accounting for up to 3.5 million deaths in children less than 5 years old worldwide.’ In the United States, 220,000 patients are hospitalized per year, accounting for 10.6% of admissions in this age group, whereas most of the 300 to 400 deaths per year attributed to diarrhea occur in the first year.’ The established pathogens of pediatric diarrhea are listed in Table 1. In up to 40% of presumed acute infective diarrhea, no pathogen is isolated. Epidemiology is important in predicting likely pathogens, especially in terms of age, geography, season, water source, travel, and day-care exposure. The current approach to diarrhea in childhood is to establish whether we are dealing with a secretory or osmotic (malabsorptive) process. The immediate problem is to avoid or correct dehydration. High fever, particularly with bloody diarrhea, requires investigation for possible bacteremia and sepsis. Furthermore, documentation of the causative organism, bacterial, viral, or parasitic, is mandatory in infants. The susceptibility of young infants to bacterial toxins is becoming more apparent. On the one hand, the greater susceptibility of infants to enterotoxigenic Escherichia co/i is due to higher density of guanylate cyclase-associated receptors in the small intestinal enterocytes. On the other hand, Shiga toxin of shigellosis does not affect the infant as much because of underdeveloped glycolipid receptors on the enterocytes. Unusual organisms in the stool, like persistent cryptosporidium might raise the suspicion of AIDS and immunodeficiency. Nonspecific diarrhea may be a symptom of infection elsewhere, such as pneumonia, otitis media, and appendicitis, and is generally not dehydrating unless it coexists with anorexia and vomiting. Noninfectious causes such as dietary indiscretion, especially overfeeding and the use of large quantities of fruit juices that include fructose, sorbitol, and sucrose may precipitate diarrhea, and food poisoning from preformed toxins tend to cause brief problems (botulism an important exception) and may be suspected from the history. Ingestion of drugs and chemicals, for example, heavy metals, should always be considered. Many proprietary antipyrexial elixirs contain sig-