Infections, malignancies and autoimmune diseases are the most important causes of fever of unknown origin.A case report of a 80-year old patient is described, who was admitted to our hospital because of fever lasting more than 2 weeks, weight loss and lack of appetite. Physical examination did not provide any relevant information, laboratory tests revealed an elevation of inflammation markers and anemia. Serological tests for infectious and autoimmune diseases and cultures of stool, blood and urine were all negative. Imaging did not show any pathological findings, colonoscopy and gastroscopy were macroscopically normal. Surprisingly, histology showed massive giardiasis of the duodenum. After initiation of therapy with metronidazol, fever and inflammation markers declined and the patient could be discharged from hospital without complaints on the 15(th) day after admission. Giardia lamblia is one of the most common intestinal pathogens worldwide. Infection can cause acute diarrhea, but may also be responsible for chronic abdominal complaints or may stay asymptomatic. To our knowledge, giardiasis has not been described as differential diagnosis of fever of unknown origin so far. In synopsis of clinical presentation and outcome after antibiotic therapy, we postulate that Giardia lamblia was the relevant cause of fever and weight loss in this case. In patients presenting with these symptoms, Giardia lamblia should be considered as differential diagnosis.