We study with a prospective design all reactive arthritis diagnosed at the Service of Rheumatology in Bellvitge Hospital during a thirty months period, from August 1985 to February 1988; in order to describe its clinical features, etiological factors and prognosis. Thirty patients who fulfilled the admission criteria were admitted to the study. A previously designed protocol of clinical, serological and microbiologic study was applied to all. A microorganism was identified in 21 patients. In 14 cases it was Yersinia enterocolitica, in 4 Chlamydia trachomatis, in 2 Salmonella enteritidis and in 1 a group A beta-hemolytic Streptococcus. The averaged length of the disease was 7.6 +/- 8 months.