Appendicitis and diverticulitis of the colon are the two main causes of febrile acute abdomen in adults. Diagnosis from imaging (ultrasound and CT) is usually easy. However, an imaging procedure which is not suitable for the clinical situation and an examination performed with the wrong protocol are sources of error and must be avoided. Anatomical variants, inflammatory cancers, complicated forms (perforation, secondary occlusion of the small intestine, peripheral abscesses, fistulae, pylephlebitis, liver abscesses) and associated signs related to a peritoneal inflammatory reaction (reflex ileus, reactive ileitis or salpingitis) can also lead to a wrong diagnosis.