What a joy to the clinician to find the arcane skills of research scientists directed to such matters as bloating, flatulence, cramps, and diarrhea! It is now clear that these symptoms can be caused by milk and other foods containing lactose in many persons deficient in the intestinal enzyme lactase. Rapidly moving research has localized lactase, the enzyme that splits lactose into glucose and galactose, and other disaccharidases to the intestinal epithelium. Peroral biopsy specimens of jejunal mucosa can be assayed accurately, providing the basis for a new field of investigation. As Kern and Struthers point out in this issue (p 927), when mucosal lactase is deficient, lactose is absorbed poorly from the intestinal lumen. Its osmotic activity results in great influx of water through the intestinal wall—more than 1,400 ml in one experiment—followed by cramps and watery diarrhea. As unabsorbed lactose reaches the lower intestine, rapid fermentation takes place,