The disaccharide lactose, the principal carbohydrate of animal milks, requires the enzyme lactase to split it to glucose and galactose. Undigested lactose passes to the colon where fermentation produces hydrogen and short-chain fatty acids that can cause abdominal distention, pain and sometimes diarrhea. Persistence of intestinal lactase after early childhood, is inherited as a highly-penetrant autosomal dominant genetic characteristic. On the basis of a review of over 560 references, all available data on the primary loss of intestinal lactase in Latin American populations are presented in tabular form. Prevalence of lactose non-digesters in Latin American populations ranges from 45% to 100%. However, this is not a reliable predictor of the acceptability of milk and milk products containing lactose. Milk is being used successfully for the supplementary feeding of children worldwide, and most lactose non-digesters can tolerate at least 240 ml of milk or the lactose equivalent in other products. Lactose maldigestion does not interfere with the absorption of the protein and essential micronutrients in milk. Information is provided on the lactose content of milk and milk products, on the usual milk consumption of Latin American populations, and on worldwide experimental and field observations of milk acceptability. Both adaptation to continued use of milk and milk products and relationships of milk use to various disease states in which intestinal lactase activity may be reduced are discussed. Some types of yoghurts are better tolerated because of the lactase activity of the bacteria used in their fermentation. For unusually intolerant individuals commercial enzyme preparations are available for addition to milk products but for most persons the additional cost is unnecessary.