Clinical course, serological response and effect of antibiotic treatment were evaluated in 34 patients with uncomplicated enterocolitis and in 27 patients with reactive arthritis (ReA) due to Yersinia enterocolitica. Patients participating in this prospective multicentre trial were randomised to treatment or no treatment with antibiotics. Only 3 (11%) of the patients who later developed ReA asked for medical care because of intestinal symptoms and fever and they all developed arthritis within 2 days after admission to hospital, i.e. before an etiological diagnosis was obtained. Patients with ReA had a history of milder intestinal symptoms than patients with uncomplicated enterocolitis. The peak IgA titer to Y. enterocolitica, as measured by enzyme linked immunosorbent assay, was higher in ReA patients and it was not affected by the presence of HLA-B27 antigen. On the other hand, untreated patients with uncomplicated enterocolitis had the longest duration of IgG antibodies. The duration of IgG antibody response was shortened in uncomplicated enterocolitis patients treated with antibiotics, but not in ReA patients. Treatment did not influence intestinal or ReA symptoms. It is concluded, that patients with uncomplicated enterocolitis due to Y. enterocolitica differ in intestinal symptoms and serological response, compared with patients who develop ReA. These parameters could however not be used to predict the development of arthritis in individual patients.