The aim of this 100-patient prospective study was to compare three types of investigations in the diagnosis of acute appendicitis: initial clinical examination, initial appendicular ultrasound examination, and final results of repeated clinical examinations carried out within the first 48 hours. Final results included initial evaluation, obvious clinical results and definitive results from patients under 48 hour observation for uncertain diagnosis. Clinical initial or appendicular ultrasound examinations were poorly contributive to diagnosis (overall accuracy: 76 percent). On the contrary, repeated clinical examinations yielded a 100 percent sensitivity, a 94 percent specificity, a 94 percent positive predictive value, a 97 percent negative predictive value and a 97 p. 100 overall accuracy. No morbidity was noted in observing patients for 48 hours. Unwarranted appendectomies were carried out in 3 percent of cases. Ultrasonography of the appendix seems useful for the differential diagnosis of acute appendicitis.