A surveillance of the clinical manifestations, course and outcome of 261 patients with gastrointestinal infection by Yersinia enterocolitica between 1982 and 1991 was carried out. Acute uncomplicated enteritis was diagnosed in 169 patients, complicated enteritis in 37, appendicular syndrome in 33, ileitis in 8 and colitis in 14. Children (age < 16 years, n = 105) presented most often with mild enteritis, young adults (age 16-25 years, n = 47) with enteritis or appendicular syndrome, adults (age > 25 years, n = 109) had significant risk for developing serious enteritis, ileitis and colitis. Complications included reactive arthritis, septicaemia, lymphadenitis, disturbed liver functions and erythema nodosum. Four patients died of generalized peritonitis. Diagnosis was established by positive culture in 207 patients. Another 54 patients were diagnosed by having at least two other positive tests: serum agglutinins, specific IgA and IgG antibodies to Yersinia outer membrane proteins (Yops) or antigen detection in biopsies. Culture alone was sufficient to diagnose uncomplicated enteritis, antiYops serology appeared to be very useful in diagnosing patients with other manifestations of yersiniosis. The majority of the infections were caused by serotypes O3 and O9 while unusual serotypes were associated with advancing age and colitis.