A total of 852 healthy Chinese children from 1 month to 14 years of age were investigated for lactose malabsorption using the breath hydrogen test. A lactose load of 2 g/kg was administered to 528 children and 1 g/kg to another 324 children. The incidence of lactose malabsorption was significantly different among various age groups and between the two different lactose dosages. While the incidence of malabsorption increased strikingly from the age of 3 to the age of 5 years, the age effect remained constant after the age of 6 years. The lactose load of 2 g/kg induced a higher incidence of malabsorption in study subjects than 1 g/kg did, and the dose effect increased for those aged below 5 years and decreased above 5 years. Small-intestinal lactase was also studied in 34 children aged from 2 months to 15 years. The lactase activity was highest in children 2 years old or younger, gradually decreasing with increasing age. In conclusion, lactase activity in Chinese children decreases gradually after 3 years of age, a finding that correlates well with the results of the breath hydrogen test.