While about 50 million Americans malabsorb lactose, the colonic metabolism of this disaccharide may prevent the symptomatic state known as lactose intolerance. Elucidation of the clinical importance of lactose malabsorption requires comparison of symptoms after ingestion of lactose with those following an identical appearing lactose-free control. This paper reviews the extensive literature concerning lactose-induced symptoms and the value of lactose digestive aids. Poorly controlled studies have suggested that a cup of milk results in appreciable symptoms in the majority of lactase-deficient subjects. In contrast, controlled trials in unselected lactose malabsorbers of subjects claiming severe lactose intolerance indicate that symptoms from a cup of milk are no greater than that with a lactose-hydrolyzed control. An increasing fraction of subjects experience symptoms as the lactose load is increased, with the majority having symptoms when the equivalent of 1 L of milk is ingested as a single dose. Further studies are required to determine the tolerance to several cups of milk taken throughout the day. Available digestive aids include pre-hydrolyzed milk and lactase preparations that can be added to milk (which is then incubated) or ingested with milk. While these products are effective in reducing symptoms, it should be emphasized that there appears to be no need for these preparations when the dosage of milk is limited to one cup per day.