The recent discovery of the mutated gene responsible for Familial Mediterranean Fever (FMF) is supposed to facilitate its diagnosis which up till now is a clinical one because there are no specific laboratory tests. The sensitivity of genetic testing is limited because these tests search only for known mutations. In this case report we describe a patient with periodic abdominal pain in whom the diagnosis of FMF was wrongly discarded because of lack of a durable effect of colchicine and negative genetic testing. Diffuse peritoneal inflammation was nicely demonstrated by a FDG-PET (fluoro-deoxy-glucose positron-emission tomography) performed during a typical crisis. We discuss the possible diagnostic pitfalls and conclude that a crisis-PET might upgrade the level of diagnostic certainty in equivocal cases.