Patients with yersinia triggered reactive arthritis were double blind randomly allocated to receive treatment with ciprofloxacin 500 mg twice daily orally or placebo during three months. The diagnosis was made by serology (specific IgA and IgG antibodies to yersinia outer membrane proteins (yops)), positive culture, and/or demonstration of Yersinia enterocolitica antigen in colon biopsy specimens. Patients were evaluated monthly during and after treatment up to 12 months. Of 18 patients enrolled, all could be evaluated for safety, 16 for efficacy. There was a tendency towards faster remission and relief of pain in those receiving ciprofloxacin. Y enterocolitica was eliminated from the gut associated lymphoid tissue in six of seven patients receiving ciprofloxacin compared with none of nine patients receiving placebo. Patients receiving placebo had more and prolonged circulating IgA antibodies against yops than patients treated with ciprofloxacin.