This Opinion of the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) deals with lactose thresholds in lactose intolerance and galactosaemia. LACTASE DEFICIENCY AND LACTOSE INTOLERANCE: Primary lactase deficiency, also referred to as lactase-nonpersistence (LNP), is genetically determined and a normal, developmental phenomenon characterised by the down-regulation of lactase activity. In adults with LNP, undigested lactose reaches the colon where it can elicit symptoms of lactose intolerance. Lactose tolerance varies widely among individuals with lactose maldigestion. A single threshold of lactose for all lactose intolerant subjects cannot be determined owing to the great variation in individual tolerances. Symptoms of lactose intolerance have been described after intake of less than 6 g of lactose in some subjects. The vast majority of subjects with lactose maldigestion will tolerate up to 12 g of lactose as a single dose with no or minor symptoms. Higher doses may be tolerated if distributed throughout the day. GALACTOSAEMIA: Galactosaemia is caused by three different genetic enzyme defects in the metabolism of galactose. Severe galactosaemia, if untreated, is accompanied by a potentially fatal impairment of hepatic and renal function and with cataracts in the newborn and the young infant. The dietetic principle in the management of all types of galactosaemia is the elimination of all sources of galactose, including human milk, as far as possible. Dietetic management is started with lactose free infant and later follow-on formulae with a lactose content ≤10 mg/100 kcal. In older infants, children and adults, foods containing milk or milk products or lactose as an ingredient must be avoided, as far as possible, so that the overall daily lactose intake will be about 25 mg/100 kcal. A precise threshold for galactose/lactose intake below which adverse effects are not elicited cannot be given.