Abstract: Four members of a farnily（father,mother,daughter,5-year-old granddaughter）fell ill one afteranother in two weeks.They had signs and syrnptoms of respiratory infection including cough expectoration,chest pain,auscultatory rales,with or without fever.Pulmonary infiltrations were seen on the chest film,and diagnosis of pneurnonia was made,Sera were tested for immunoglobulin M and G （IgM and IgG）anti-bodies against chlamydia pneumonitqe antigen,using the microimmunofluorescence（MIF）technique（in thecases of the father,mother and daughter）.A fourfold rise of IgG antibody titer was found in the case of fa-ther.The IgG titer was 1:1024 in caxes of mother and daughter,therefore acute of recent infection withchlamydia pneumoniae was considered.Upon clinical trail with antibiotics the adult patients（father,motherand granddaughter）responded only to tetracyclines and macrolides antibiotics,According to epidemic analy-sis.an outbreak of chlamydia pneumonae infection in the family was confirmed.Literatures on chlarnvdiapneumoniae infection were reviewed and briefly discussed.