The rate of yersiniosis in patients with acute abdominal disease was studied in a 16-month prospective investigation in 1972-1973 of 205 acutely ill patients referred to a surgical clinic of a Copenhagen City hospital with complaints of abdominal pain suggestive of appendicitis. Yersinia enterocolitica, biotype 4, was isolated from 11 patients (5.4%), 8 of whom were children. Yersinia was grown from faeces in 8 cases and from appendix of all 9 patients operated upon. Rising or falling agglutinin titres larger than or equal to 100, indicative of yersiniosis, were found in 22 patients (10.7%) including all bacteriologically verified cases. Eight additional patients (3.9%) had less significant titres larger than or equal to 100, suggestive of recent or present infection. 28 patients (13.7%) had insignificant titres, including 3 with antibodies against serotype 9. In all cases except these 3, antibodies were against Y. enterocolitica, serotype 3. A differential diagnosis between Y. enterocolitica infection and other types of appendicitis could not be made within this highly selected group of patients using available clinical data. All cases were rather mild and self-limiting. It is suggested that in future epidemiological and other studies of yersiniosis, early bacteriological and serological examinations be carried out.