a b s t r a c tWe describe the case of an 18-yr-old male under anti-TNF treatment for Crohn's disease for more than 8 months. He developed fever and biological inflammatory syndrome without absolutely no accompanying sign or symptom or paraclinical abnormality despite extensive work-up performed in the context of his immunocompromised state. Symptoms disappeared after 10 days and a diagnosis of Puumala infection was made retrospectively on a serological basis. The case illustrates that anti-TNF treatment does not worsen the course of Puumala infection and could even be associated with a milder clinical picture.