The differential diagnosis of chronic diarrhea varies markedly with age. In infants, it is usually a problem with formula intolerance. Because there is up to a 50% crossover intolerance between milk and soy, the infant should be given an extensively hydrolyzed formula. If such intervention is delayed, he or she may develop intractable diarrhea of infancy. Most affected toddlers have either irritable colon of infancy or protracted viral enteritis with low-grade mucosal injury and are consuming hypertonic feedings. In either case, institution of a high-fat, low-carbohydrate diet that includes whole milk often results in significant improvement. Dietary lactose rarely is a problem. A likely cause of diarrhea among children and adolescents is inflammatory bowel disease. With the exception of toddlers, chronic diarrhea suggests the presence of significant organic disease.