Lactose breath test have been performed in healthy newborns, in whom breath hydrogen excretion was preliminarily demonstrated by lactulose breath test. 27.5% of newborns showed positive lactose breath test, with no differences related to sex and gestational age, while a higher percentage of positive test has been found in < 2500 g and formula-fed neonates compared, respectively, to > or = 2500 g and breast-fed ones. In healthy newborns, without any clinical signs of lactose intolerance, it is possible to demonstrate hydrogen in expired air; this is not equivalent of lactose malabsorption, but, instead, depends on the phenomenon of colonic fermentation that allows energy and nutrient absorption.