Objective: Unusual clinical course Background: Protein-losing enteropathy is a rare cause of hypoproteinemia. Erosive and non-erosive gastrointestinal diseases as well as vascular disorders that result in increased central venous pressure or mesenteric lymphatic obstruction may result in protein loss via the gastrointestinal tract. Case Report: We present the case of a 50-year-old man with protein-losing enteropathy, who had profuse diarrhea, abdominal pain, lymphadenopathy, fever, and a weight loss of 10 kg in the preceding 2 months. Extensive work-up revealed infection with Giardia lamblia. We review clinical signs and symptoms, laboratory findings, and imaging studies, and discuss potential pitfalls in establishing the diagnosis. Conclusions: To the best of our knowledge, this represents one of the few published cases of intestinal giardiasis as a cause of protein-losing enteropathy in an immunocompetent adult. The diagnosis of lambliasis should be based on a combination of stool cultures and serum serology, and in cases of high clinical suspicion, an endoscopy and biopsy of the upper GI tract is recommended.