Post-enteritis reactive arthritis-mainly caused by Yersinia enterocolitica-has succeeded rheumatic fever as the most frequent postinfection arthritis in Northern and Central Europe. Aetiologically important are infective diseases-produced by "arthritogenic" bacteria--as a trigger, and genetic predisposition, indicated by a high incidence of HLA-B27 (80%). Over a period of 5 years 51 cases of post-enteritis reactive arthritis were diagnosed in our clinic; the patients--after the first examination--were followed up for several years. The typical clinical findings initially were an asymmetric oligoarthritis, mainly affecting knees, ankles and toes. Transitory low-back pain and heel pain were the most important concomitant findings. Transient liver involvement was indicated by pathological laboratory tests in 12 cases. Acute primary disease was determined at latest within one year (average duration 6.5 months). In the follow-up period 7 patients developed symptoms of a chronic seronegative spondylarthritis. Thus, post-enteritis reactive arthritis may often represent the first manifestation of a chronic disease of the seronegative spondylarthritis group, e.g. Reiter's syndrome.