A very rare and extremely serious illness, inherited in an autosomal recessive pattern (Diekmann 2015, Torniainen 2009, Järvelä 1998, Levin 1970). The congenital absence of intestinal lactase prevents the milk lactose from hydrolyzing leading to very serious diarrhea symptoms from the first days of life (Torniainen 2009, Savilahti 1973, Levin 1970). Although treatment consists of a lactose-free diet and, therefore, it is not possible to breastfeed, there is a reported case of feeding a baby affected by this disease with hydrolyzed milk from a milk bank with added lactase: 900 units of lactase activity in 200 ml of milk (Similä 1982). Breastfeeding is not contraindicated in secondary temporary intolerance to gastroenteritis.