Reactive arthritis is an aseptic inflammation of synovium, tendons and fasciae triggered by an immunological reaction to a distant-site infection. It is usually asymmetric. It develops about 4 weeks after a prior infection of the genitourinary system (most often caused by Chlamydia trachomatis or Neisseria gonorrhoeae) or the gastrointestinal tract (most often: Shigella enteritidis, Salmonella typhimurium or Salmonella enteritidis, Campylobacter jejuni, Yersinia enterocolitica). A typical triad of symptoms includes: arthritis, conjunctivitis, and urethritis. Men aged 20-50 are most commonly affected. A considerable percentage of patients with reactive arthritis has the HLA-B27 antigen. Presence of the antigen heralds a more severe course of the disease and occurrence of articular complications. The disease may have a neoplastic course, sometimes it subsides without chronic consequences, and in some cases it evolves into other spondyloarthropathies.