The relationship between breath gas and lactose intolerance symptoms is studied in the subjects with lactose intolerance and lactose mal-absorption. The breath gas samples are collected for 6 hours after consumption of 25 g 13C-lactose. Expired H2 concentration and 13CO2 abundance at each detecting time point is measured by gas chromatography and continuous flow Gas Isotope Mass Spectrometry respectively. The cumulative amount of breath H2 and 13CO2 for 6 hours are calculated, and the abdominal symptoms are recorded by the questionnaire for 12 hours. The results show that the H2 peak is significantly higher in the intolerance group than that in the mal-absorption group(P < 0.01). The cumulative amount of breath H2 during 6 hours is significantly higher in the intolerance group than that in the mal-absorption group (P < 0.01). The oral-colon transit time (OCTT) is negatively related to the lactose intolerance symptom scores (r = -0.705). There are no significant differences for the amount of 13CO2 excretion at each detecting time point and cumulative percentage of the expired 13CO2 between the intolerance and the mal-absorption groups. It is suggested that the cumulative breath H2 amount is associated with the severity of lactose intolerance, which can reflect the degree of lactose hydrolysis in the small intestine, while there is no such relationship between the amount of expired 13CO2 and the severity of lactose intolerance.