A rare, recessively inherited disease, (around 1 in 75,000 births). Its importance lies in the disastrous consequences of it being overlooked, and results from the deficiency of an ENZYME essential for the metabolism of GALACTOSE. Normal at birth, affected infants develop jaundice, vomiting, diarrhoea, and fail to thrive on starting milk feeds. If the disorder remains unrecognised, liver disease, cataracts (see EYE, DISORDERS OF) and mental retardation result. Treatment consists of a lactose-free diet.