Objective To study the association between house dust endotoxin levels and the airway responsiveness as well as allergen sensitization in adolescence in the urban and rural areas of Guangdong province.Methods Middle school students,aged 13-14,188 (male 109,female 79) from Guangzhou,129(male 70,female 59) from Conghua with positive responses,and 231 (male 125,female 106) and 307(male 145,female 162) of their respective reference students are selected from questionnaire prescreening.A detailed questionnaire,the lung function test,histamine bronchial provocation test,skin prick test and peripheral blood eosinophil count are undertaken.Seventy-six students from Guangzhou and 80 from Conghua were selected randomly for mattress dust sample collection to detect the levels of endotoxin.Results The prevalence of wheeze symptom,diagnosed asthma,nasal symptoms and allergic rhinitis of Guangzhou was significantly higher than that of Conghua (P＜0.0001),with 28.6％,27.7％,66.0％,46.4％ for Guangzhou respectively,and 5.0％,2.5％,31.0％,6.2％ for Conghua respectively.The positive rates of bronchial provocation test and skin sensitization against Dermatophagoides pteronyssinus(Der.p),D.farinae (Der.f),cat and dog allergen in the students of Guangzhou were significantly higher than that of Conghua (P＜0.0001).The positive rate of cockroach of Conghua was significantly higher than that of Guangzhou (P＜0.0001).The levels of endotoxin (amount,concentration and load) of samples from Conghua were significantly higher than that from Guangzhou (Conghua:6452 EU,10.95 EU/mg,1794 EU/m2 ;Guangzhou:1591 EU,6.45 EU/mg,508.80 EU/m2,respectively).PD20-FEV1 was correlated positively with endotoxin amount(r=0.174,P＜0.05).The diameters of the skin wheal size for Der.p and Der.f were correlated inversely with endotoxin amount and load (P＜0.001).Conclusion The prevalence of wheeze symptom,diagnosed asthma,nasal symptoms and allergic rhinitis in 13-14 year-old adolescent of Guangzhou was significantly higher than that of Conghua.Cockroach was the second major allergen of Conghua followed by house dust mite.The level of endotoxin was correlated positively with PD20-FEV1,but correlated inversely with skin sensitization to Def.p and Der.f.The study suggests that environmental endotoxin may have effect to protect children from developing allergic diseases.