Abstract: Ahistorical co hor-tstudy was carried out by comparing levels of sero-antibodies against Legionella pneumnophila of 414 wor kers who were taken as the exposure-group under central air conditioning systems with 414 controls from the general poputation as the non-exposure group.The results show ed that the positive rates(person-stime)of sero-antibodies against Lp1-14 from both of above two groups were 11.5j and 2.1 j respectively.The positve rate of the former was rather higher than that of the latter.(χ²=14.2 P<0.01,RR=5.48,AR%=81.8%).By means of stratified analysis,it was shown that each of positive rate of different age groups of the exposure group was higher than that of the non-exposure group,with the most differentin<35 agegroup(χ²=14.4,P<0.01,RR=17.0,AR%=94.1%).As compared with the non-exposure group,the titer of Lp1,Lp12, and Lp14 of the exposure gorup were significantly hig her than that of the non-exposure group.However,it was also shown that,by tendency of examination,the positive rate in exposure group was no long erincrease with exposure time went on(χ²= 0.59,P=0.75).The results suggested that the central ai-rconditioning system should be a potential risk factor for legionnaires.disease.