We present the case of a rare cause of fever of unknown origin (FUO). FUO is challenging for patients as well as for physicians as there are more than 200 differential diagnoses of FUO (1,2). Pointing out a diagnosis often requires numerous noninvasive and invasive procedures that sometimes even fail to explain the fever. Our patient was admitted twice to our hospital due to remitting fever rising up to 40 degrees C without any subjective discomfort. At the first presentation no clinical focus could be identified. This included the examination of multiple blood and urine cultures, serology, autoimmune serology, transesophageal echocardiography, CT-scan of the lung and the abdomen, and bone scintigraphy. Elevated C-reactive protein (268 mg/l) decreased spontaneously and fever disappeared after 4 weeks. However, the patient was re-admitted 4 months later with identical symptoms. Multiple blood and urine cultures, serology, bone marrow examination, CT-scan of the lung and the abdomen, esophago-gastro-duodenoscopy and colonoscopy still showed no pathological findings. MRI-scan of the abdomen identified a liver tumor of 3.3 cm in diameter in segment 6 without typical signs of an adenoma, focal nodular hyperplasia or hepatocellular carcinoma. Biopsy of the suspect liver lesion revealed an inflammatory myofibroblastic tumor (inflammatory pseudotumor). After surgical resection of the tumor elevated inflammation markers as C-reactive protein normalized and fever disappeared. One year after surgery no more episodes of fever re-occurred. An inflammatory myofibroblastic tumor of the liver can be a rare cause of fever of unknown origin. MRI-scan can be an additional imaging tool to identify previously not recognized liver tumors.