In cases of clinical suspicion of an acute appendicitis sonography uncovers another disease as the real cause of the symptoms in about one-quarter of such cases. Bacterial ileocaecitis is most frequently diagnosed (11.6% of N = 786). In special bacteriological stool cultures, Yersinia enterocolitica and Campylobacter jejuni were identified, whereas Salmonella enteritidis was a rarer finding. The typical sonographic manifestation of bacterial ileocaecitis compared against Crohn's disease of the ileocaecal region is described. These two diseases can be differentiated against each other by means of sonography; likewise, it is also possible to distinguish them from appendicitis. Since stool cultures--which are not always prepared if diarrhoea is only mild or completely absent--are received relatively late in acute cases, knowledge of the sonographic manifestation of bacterial ileo caecitis can help save many an unnecessary laparotomy.