Find the article at: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1005260Abstra… examine whether reactive arthritis (ReA) known to occur after a urogenital infection with Chlamydia trachomatis can also follow an infection with Chlamydia pneumoniae, a recently described species of Chlamydiae that is a common cause of respiratory tract infections. METHODS--Specific antibodies (microimmunofluorescence test) and lymphocyte proliferation to C trachomatis and C pneumoniae in paired samples of peripheral blood and synovial fluid were investigated in 70 patients with either reactive arthritis (ReA) or undifferentiated oligoarthritis (UOA). RESULTS--Five patients with acute ReA after an infection with C pneumoniae are reported. Three had a symptomatic preceding upper respiratory tract infection and two had no such symptoms. In all patients a C pneumoniae-specific lymphocyte proliferation in synovial fluid and a high specific antibody titre suggesting an acute infection was found. CONCLUSION--C pneumoniae needs to be considered a new important cause of reactive arthritis.