To assess the possible etiological link between genital infection due to Chlamydia trachomatis (CT) and Reiter's syndrome (RS) 24 men and 2 women with typical RS and 5 women with signs suggestive of RS (SRS) were examined. CT was isolated by irradiated McCoy cell culture technique and chlamydial indirect immunofluorescence serology was applied. Chlamydial isolation from the urethra was positive in 5 patients as well as from synovial fluid of the knee in one man. The serology was positive (titre greater than or equal to 64) in 17 (55%) of the 31 patients, including all 7 women. HLA-B27 antigen was detected in only 15 of 27 patients (55%). All except one of the 12 patients negative for HLA-B27 antigen had positive chlamydial serology. Among the patients were two married couples, both negative for B27 antigen and positive for chlamydial serology. The case report of the one couple, in which the wife had positive urethral chlamydial isolation, is given. Sexually acquired RS might easily be overlooked in women and be misdiagnosed as seronegative arthritis. The classification for RS associated with CT infection is discussed. The simultaneous treatment of the infection in sexual partners is emphasized.