Summary: Lactose is the main and important carbohydrate of breast milk, which has important functions and its role is emphasized in the proper development of infants and young children. Hypolactasia is a state of decreased lactase activity, often leading to lactose intolerance. Cow's milk allergy is sometimes equated with lactose intolerance, however, lactose intolerance is most often caused by mechanisms that do not have an immune basis and is usually not associated with an allergy to cow's milk proteins. It is important to make a correct diagnosis so as not to expose your child to unnecessary stress from an inappropriate elimination diet. Cow's milk allergy and lactose intolerance are two different diseases from the point of view of pathogenetic mechanisms. An important issue may be the coexistence of hypolactasia with immunological hypersensitivity to cow's milk proteins, although it is currently rare. An allergy to cow's milk proteins is not the same as lactose intolerance, therefore the elimination of lactose is justified only in the case of lactose intolerance. The paper presents the importance of lactose, hypolactasia and its forms, methods of diagnosis and treatment of lactose intolerance and allergy to cow's milk proteins.