An outbreak of upper respiratory infection (influenza-like syndrome) took place among the staff members of a large office building in Beijing, June, 1997. To understand the cause of the outbreak, with a case-control study with serological antibodies detection against serogroup 1 to 14 of Legionnaires' disease bacteria (LDB) and bacteriological test and culture on samples collected from the air-conditioning system of the office building was performed. The findings showed that the attack rates of tonsillitis and upper respiratory infection were 61.86% and 34.62% respectively. The relation was statistically significant between the symptoms of fever and tonsillitis in the cases (chi 2 = 77.88, P < 0.01). The positive rates of LDB antibodies on serogroups 9, 10, 12, 14 were 45.94% in the cases of upper respiratory infection and 3/15, 6/17, 0/15, 1/16 respectively in the four controls groups. Condensied water samples from an air-conditioning system showed that Legionella Pneumophila was positive. Evidence showed that it was an outbreak resulting from LDB serogroup 9 and 12, accompanied by upper respiratory infection related to air-conditioning system contaminated by LDB. This was the first report Lp infection associated with an air-conditioning system in China.