We describe a three-year-old boy with clinical findings of infectious mononucleosis, but no serologic proof in the beginning of his disease. The subsequent course was complicated by fatal pulmonary complications compatible with an adult respiratory distress syndrome (ARDS). Repeat serologic tests for Epstein-Barr virus finally could confirm the diagnosis of infectious mononucleosis. Thus, ARDS has to be added to the possible rare causes of fulminant, fatal cases of infectious mononucleosis.