Background. The aim of this study was to contribute to the insight of the role of the infectious agent in ethiopathogenesis of the Reiter’s syndrome development, which could directly influence the choise of treatment of these patients. Methods. Eighteen patients with urogenital form of the Reiter’s syndrome and 16 controls (6 with rheumatoid arthritis and 10 with pigmented villonodular synovitis) were included in the study. In all patients standard laboratory analyses of the blood, urine and stool were made; antibody titer to Chlamydia trachomatis and Ureaplasma urealyticum was determined in synovial fluid and serum; isolation of Chlamydia trachomatis and Ureaplasma urealyticum in urethral, cervical and conjunctival swabs, as well as in prostatic and synovial fluid, was also made. HLA typing was done, too. Chlamydia was isolated in the McCoy cell culture treated with cycloheximide while Ureaplasma was identified according to its biochemical properties grown on cell-free liquid medium. Results. Chlamydia trachomatis was isolated from the synovial fluid of 4 patients with Reiter's syndrome 22.2%), while Ureaplasma urealyticum was isolated in 7 of them (38.9%). These microorganisms were not found in any synovial fluid of the control group patients. Conclusion. Presence of these bacteria in the inflamed joint might be an important factor in etiopathogenesis of this disease, and it supports the hypothesis that arthritis in Reiter's syndrome is probably of the infectious origin.