Sample size calculation The sample size calculation was performed according to the methods by William D. Dupont and Walton D. Plummer (PS Power and Sample Size Calculations). For the sample size calculation the investigators performed a retrospective cohort analysis in 2008 and 2009 in 5 hospitals in the Netherlands (Wilhelmina Ziekenhuis Assen, Nij Smellinghe Ziekenhuis Drachten, Universitair Medisch Centrum Groningen, Medisch Centrum Leeuwarden and Martini Ziekenhuis Groningen). During this period 843 patients were operated for the suspicion of acute appendicitis with a diagnostic laparoscopy, 9% of the removed appendices turned out to be normal (appendix sana). In order to half the negative appendectomy rate with a power of 0.8 and an alpha of 0.05, 544 patients in the prospective group are needed. Our previous prospective analysis (the LAPP study) and the retrospective data showed that approximately 15% of the included patients will have another diagnosis (Crohn's disease, ruptured ovarian cyst etc.), additionally the investigators expect another 15% of exclusions because for example no clear laparoscopic view of the appendix (figure 1). Therefore the investigators need 778 patients in our multicenter prospective validation study.