Mesenteric panniculitis is an underdiagnosed inflammatory condition of unknown etiology that involves the mesenteric adipose tissue. Clinical symptoms are not specific (fever, abdominal pain, or nausea), so diagnosis is not simple. We report a case of 48-year-old man with abdominal pain and fever not responsive to antipyretics and antibiotics for 2 months, who underwent an F-FDG PET/CT after a nondiagnostic CT scan. PET/CT has proven to be a useful method for the diagnosis of mesenteric panniculitis in this patient, showing a uniformly high uptake of FDG in the mesenteric adipose tissue of the mesocolon, mesoappendix, mesosigmoid, and mesorectum.