Lactose malabsorption was investigated in 169 Chinese children aged between two and 16 years using the breath hydrogen test. The challenge was either lactose solution (1 g/kg) or cow's milk (10 ml/kg). Overall, 68% of the children showed a significant increase in breath hydrogen following the lactose challenge while only 17% showed an increase after the cow's milk challenge and 13% after both challenges. The number of malabsorbers increased significantly (p less than 0.001) with age and no associated gastrointestinal symptoms or signs were found in any of the children following the challenges, suggesting a gradual and partial loss of intestinal lactase activity. We conclude that the prevalence of lactose malabsorption in Hong Kong children is very high using the standard lactose tolerance test but when a more realistic amount of lactose and a natural medium such as a glass of milk is used as the challenge, the number of malabsorbers becomes small and clinically insignificant.