The relationship between primary lactase deficiency, the amount of lactose in the diet, and symptoms of intolerance continues to be debated. Primary adult lactase deficiency is common with a worldwide occurrence of near 70%. Lactase-deficient individuals malabsorb lactose but may or may not show intolerance symptoms. The development of symptoms appears to depend on the dose of lactose ingested, whether it is accompanied by a meal or other food, rate of gastric emptying, and small intestine transit time. Lactose loads of 15 g or greater produce symptoms in the majority of lactase-deficient persons. However, when lactose loads of up to 12 g are fed, symptoms can be minimal or absent. Tolerance to yogurt, acidophilus milk, and other microbe-containing dairy foods has been suggested and is thought to be due to either a low lactose content or in vivo autodigestion by microbial β-galactosidase. Up to 20 g of lactose in yogurt is tolerated well by lactase-deficient persons. Associated with the consumption of yogurt is a three- to fourfold reduction in lactose malabsorption as compared with similar lactose consumption in milk. Improved lactose digestion appears due to autodigestion by microbial β-galactosidase. This enzyme may be released from yogurt culture by gastric or bile acid digestion. Feeding yogurt that was pasteurized following fermentation, with only trace amounts of microbial β-galactosidase activity, results in a threefold increase in lactose malabsorption as compared with feeding yogurt with a viable culture. However, pasteurized yogurt also is tolerated well by lactase-deficient persons, suggesting that tolerance of up to 20 g of lactose in yogurt may be independent of lactose malabsorption. The enhanced lactose absorption and tolerance observed with yogurt feeding are not apparent when unfermented acidophilus milk or cultured milk are fed.