Lactose intolerance in infants and children is a non-infectious diarrheal disease caused by low secretion of lactase enzyme, which cannot completely digest and break down lactose in breast milk or cow’s milk, also called lactase deficiency. The symptoms of lactose intolerance are mainly manifested in the following ways. The first onset is in newborns or small infants within 3 – 4 months of age, manifesting as diarrhea, the number of stools is more than 10 times a day, most children have a lot of intestinal gas, often bringing out a small amount of feces on the diaper, the stool is mostly yellow or greenish thin paste or in the form of egg white soup, and may also manifest as irritability and occasional colic. Routine stool tests are negative, and reducing sugar and pH measurements suggest lactose intolerance. When these manifestations occur, we treat the baby well with lactose-free milk, which is what we usually call diarrhea formula, and then switch back to regular milk or breastfeeding and diarrhea occurs again. For babies who are exclusively breastfed, taking lactase before each breastfeeding can also play a good therapeutic role. Congenital lactase deficiency, this is a genetic cause, very rare, it is autosomal recessive, low lactase activity at birth or lactase deficiency, babies mostly develop after breastfeeding. Most lactose intolerance is secondary to lactase deficiency, mostly due to disease, and the symptoms are reversible, and the lactase activity returns to normal as we actively treat the primary disease and cure it. Lactose intolerance is mostly secondary, and when a child has diarrhea that is not well treated aggressively, we have to consider that it may be lactose intolerance.