Milk and milk products contain high concentrations of the disaccharide lactose (galactose and glucose linked by a beta-galactoside bond). Intestinal absorption of lactose requires that the disaccharide be hydrolyzed to its component monosaccharides, both of which are rapidly transported across the small bowel mucosa. A brush border beta-galactosidase, lactase, carries out this hydrolysis. While infants virtually always have high concentrations of lactase, sometime after weaning a genetically programmed reduction in lactase synthesis results in very low lactase activity in some adult subjects, a situation known as lactase nonpersistence. Lactase nonpersistence results in incomplete digestion of an ingested load of lactose; hence lactose is malabsorbed and reaches the colon. If sufficient lactose enters the colon, the subject may experience symptoms of abdominal pain, bloating, excess flatulence, and diarrhea, a condition known as lactose intolerance (LI). Diseases of the small bowel mucosa (infection, celiac disease) may also be associated with low brush border lactase, with resultant lactose malabsorption (LM) and LI. A public health problem may arise when large numbers of individuals diagnose themselves as being lactose intolerant. However, these self-identified lactose intolerant individuals may actually be lactase persisters. Some of these lactase persisters (and even lactase nonpersisters) may mistakenly ascribe the symptoms of undiagnosed irritable bowel syndrome (IBS) or other intestinal disorders to LI. Given that the relatively nonspecific abdominal symptoms caused by IBS and LM are extremely susceptible to the placebo effect, reliable demonstration of LI requires double-blind methodology. The problem may become intergenerational when self-diagnosed lactose intolerant parents place their children on lactose restricted diets or use enzymatic replacement in the belief that the condition is hereditary. Children and adults with LI may avoid dietary milk intake to reduce symptoms of intolerance. Since the avoidance of milk and milk containing products can result in a dietary calcium intake that is below recommended levels of 1,000 milligrams (mg) per day for men and women and 1,300 mg for adolescents, osteoporosis and associated fractures secondary to inadequate dietary calcium is the perceived major potential health problem associated with real or assumed LI. Because ingesting smaller portions over the course of the day may minimize potential problems with larger acute lactose loads, the above definition of lactose intolerance may miss lactose malabsorbers who ingest smaller dosages of lactose. The prevalence of clinically important lactose intolerance requires demonstration that the quantity of lactose that subjects actually ingest (or wish to ingest) causes symptoms in placebo-controlled experiments. Treatment to reduce lactose exposure, while maintaining calcium intake from dairy products, consists of a lactose restricted diet or the use of milk in which the lactose has been pre-hydrolyzed via treatment with lactase supplements. Lactase supplements taken at the time of milk ingestion also are commercially available. This report was commissioned as background material for a National Institutes of Health (NIH) and Office of Medical Applications of Research (OMAR) Consensus Development Conference on Lactose Intolerance and Health to address the following key questions: 1. What is the prevalence of lactose intolerance? How does this differ by race, ethnicity, and age? 2. What are the health outcomes of dairy exclusion diets? 3. What amount of daily lactose intake is tolerable in subjects with diagnosed lactose intolerance? 4. What strategies are effective in managing individuals with diagnosed lactose intolerance? 5. What are the future research needs for understanding and managing lactose intolerance?