We have estimated lactose absorption indirectly by the breath H test to see if disaccharide exclusion is necessary for untreated celiac children. Lactose at 2 g/kg body weight (maximum 50 g) was administered to 42 infants and children (ranging in age from 9 months to 12 years) with flat small intestinal mucosa. Later, different amounts of lactose were given to determine the quantities tolerated and absorbed. One hundred percent of patients expired hydrogen more than 20 ppm over the baseline after an oral lactose load of 2 g/kg (maximum 50 g). Thirty-eight percent of them did not tolerate this quantity. Thirty-seven subjects aged 0-6 years absorbed and tolerated 0.5-1.5 g/kg (5-12.5 g total), and five patients aged 6-12 years absorbed and tolerated 0.5-0.6 g/kg (12-16.2 g total). We conclude that in many subjects with untreated celiac disease, lactase activity is sufficient for absorption and tolerance of the amount of lactose present in 250-300 ml cow's milk. Because of lactose's nutritional value, it should not be excluded unless necessary.